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.

Fix HttpClient-from-HttpService wrapper prematurely cancelling service promise.

The test "WebSocket pump disconnect on send" tests what happens if a send() throws DISCONNECTED while pumping. The test hung forever with socketpairs because the message fits entirely in the socket's send buffer, and thus send() actually completes before the disconnect occurs. This differs from userspace pipes, where there is no buffering, and so the send() doesn't complete until all data is received.

This could be fixed by sending a very large message, so that the send() actually blocks on the receiver. But, the problem reveals that WebSocket pumps won't notice when the destination disconnects unless the source sends some message through. Now that we have whenWriteDisconnected(), we can fix this and proactively end the pump, which also fixes the test.

In fact, we can and should propagate such disconnects backwards: if the destination disconnects, we should close the source. We can do this by adding an abort() method to WebSocket, which incidentally allows us to remove the internal AbortableWebSocket interface which was used specifically by WebSocketPipeImpl.