Various fixes for "Core IPC messaging rewrite" (PR #130)

[kinetiknz]

Various fixes found while stress testing PR #130 integrated with Gecko on various platforms.

• 25ba70ed054c30dc3e83c98614ebba316ef491ad: Consume incoming cmsg buffer correctly.
  • clone was causing processed handles to be left in the cmsg buffer, which were then ignored when trying to associate them with the incoming message in set_owned_handle.
• cc170ab91be6659ae8e1715185f327db01fedcfd: Notify EventLoop when proxy dropped, and clean up broken connections when handling wakes.
  • Resulted in a shutdown hang in the AudioIPC client while waiting for the server's CallbackClient to be removed.
• 06b8727b8e771cdeba49934066a12481773ddcf2: ipctest: 64kB stack is too small for Client Callback thread on Windows.
  • ipctest was using an unusually small stack size, resulting in an intermittent crash if the Client Callback thread stack was exhausted. This didn't show up in the old code because we bounced callback processing to the thread pool.
• 74b5f83967734ba2547da04ace9783e8b1f8f78c: Reserve appropriate buffer size in encode.
  • Bug introduced when backporting the codec.rs speed ups from my development branch (using bytes 1.1.0).
• 50ae2a6e4e2558d6dc9261ab84a78d9aff3ad1d7: Force Proxy's Sender to upgrade when created.
  • Fixes an intermittent crash in Proxy::call if the Sender happened to still be in Oneshot state.
• 1d255f7c7cc5a6f4350cd72e6e533e744549abbb: Use a separate EventLoopThread for device_collection_change callbacks.
  • Fixes a hang in the server when setting up DeviceCollectionClient in the server while processing a cubeb_register_device_collection_changed request.

This PR will be merged into the macos branch.

[kinetiknz]

I reverted the change to the dead_server test, since it's still necessary: https://github.com/mozilla/audioipc-2/blob/5527f63a529a107de25716d2b34cf6c8396505c3/audioipc/src/ipccore.rs#L817

[kinetiknz]

Thanks! I'll add the tests in a PR closing issue #135. Merging this now so I can land in Gecko.