The new `Executor` class provides an interface to run a function on some other thread's EventLoop.

`kj::getCurrentThreadExecutor()` gets a reference to this thread's executor, which can then be exposed to other threads.

When a thread requests execution of a function on another thread, the return value is returned to the requesting thread. The requesting thread may choose to wait synchronously for this return value or -- if the requesting thread has an event loop of its own -- it can get Promise for the eventual result.

Meanwhile, orthogonally, the function can either return a raw result or return a Promise; in the latter case, the Promise is resolved to completion in the executor thread and the final result is sent back to the requesting thread.

This is better than `MutexGuarded<T>::when()` in cases where the caller already has a lock -- `when()` would require unlocking, then immediately locking again (to check the predicate).

Possibly `when()` should be removed in favor of `wait()`.

This will be important in subsequent commits adding cross-thread events to EventLoop. We'll only want wait() to throw if we know no cross-thread events will ever come, but the EventPort won't know that, only the EventLoop will.

Add basic system clock APIs.

Fix filesystem API: Empty mmaps should succeed.

I had a snarky comment in the Windows code being annoyed that mapping an empty file was documented to fail... but it turns out zero-length mappings fail on Linux, too. Also my work-around on Windows didn't work (but was never tested).

This PR fixes both.

This was prompted by @KubaO observing that `capnp compile` on an empty source file failed with a cryptic mmap error.

Closes #854, @KubaO's attempt at fixing this, since the correct fix is in the filesystem API, not in the compiler.

Fixes #822

Fixes #840

Doc fix: Expand the short description of the list pointer.

I didn't bother for posix condvars since they're kind of explicitly tied to posix clocks and it would have saved like two lines of code. But for Windows it's nice to stop using QueryPerformanceCounter() directly with the complicated math.

Add per-object streaming flow control

Implement MutexGuarded::when() (i.e. condvars) on all platforms.

Note that at present I think the only way they would have happened not under-lock is if one of the pthread or syscalls failed, which should never happen. Exceptions thrown by the predicate were already always rethrown under lock. But it doesn't hurt to be safe.

Fix sendStream() failing if it can't complete immediately.

It helps to list, in one place, both ways the list size is conveyed.

And it turns out that the Windows implementation was returning too early due to rounding error. Fixed.

It turns out GetTickCount64() is only precise to the nearest timeslice, which can be up to 16ms. The imprecision caused test failures.

I think I imagined once upon a time that this would be a convenient way to deal with external interfaces that like to return nullable pointers. However, in practice it is used nowhere in KJ or Cap'n Proto, and it recently hid a bug in my code where I had assigned a `Maybe<T>` from an `Own<T>`. We can introduce a `fromNullablePointer()` helper or something if that turns out to be useful.

Consider a capnp streaming type that wraps a kj::AsyncOutputStream.

KJ streams require the caller to avoid doing multiple writes at once. Capnp streaming conveniently guarantees only one streaming call will be delivered at a time. This is great because it means the app does not have to do its own queuing of writes.

However, the app may want to use a CapabilityServerSet to unwrap the capability and get at the underlying KJ stream to optimize by writing to it directly. However, before it can issue a direct write, it has to wait for all RPC writes to complete. These RPC writes were probably issued by the same caller, before it realized it was talking to a local cap. Unfortunately, it can't just wait for those calls it issued to complete, because streaming flow control may have made them appear to complete long ago, when they're actually still in the server's queue. How does the app make sure that the directly-issued writes don't overlap with RPC writes?

We can solve this by making CapabilityServerSet::getLocalServer() delay until all in-flight stream calls are complete before unwrapping.

Now, the app can simply make sure that any requests it issued over RPC in the past completed before it starts issuing direct requests.

Also, push harder on the code generator such that `StreamResult` doesn't show up in generated code at all.

So now we have `StreamingRequest<Params>` which is like `Request<Params, Results>`, and we have `StreamingCallContext<Params>` which is like `CallContext<Params, Results>`.