Add HttpClient wrapper for limiting concurrent connections

Feature grab bag

Not sure how this slipped by for so long!

This class lets you hold a temporary value that is only safe to access under lock, while you release the lock to do some other work.

Jlee/fix mingw warnings

http-test: Replace windows-conditional block with extra poll

Seems to work reliably under Windows, hopefully also Mac.  I'm not
totally sure why the extra polling is necessary, though.

`Maybe<T>(Maybe<T&>&&)` should do a copy, not a move.

`Maybe<T&>` does not own the `T` it points to. But, before this change, initializing a `Maybe<T>` from a `Maybe<T&>` would inadvertently perform a move, stealing the `T` away from its owner.

Fix capnpc extension handling on Windows

On Windows executables needs to end with .exe.
Fix this in CMakeLists.txt and update capnp detection code accordingly.

Fix narrowing conversion

Implement InsertionOrderIndex move constructor/assignment operator

InsertionOrderIndex manages memory, but it didn't have an explicit move constructor or move assignment operator, causing segmentation faults and double frees.

Fix compile error with Array<const T>.

For trivial types, we already had this const_cast (line 622), but for non-trivial types it was missing.

Update rpc.capnp

Fix obscure optimization-blocking bug in PromiseIoStream::tryPumpFrom().

I don't have a use case, I was just reading the code and noticed it was wrong.

Fix obscure use-after-free in HttpClientAdapter.

In setPromise() we chain `task` with an error handler that accesses ResponseImpl, so that means `task` needs to attach a reference to RespnoseImpl... ugh.

Typos in comments

Make HttpServer detect client disconnect and cancel request processing

Skip whenWriteDisconnected() tests on Cygwin, where they don't seem to work -- the promise never resolves.