When the PlatformConfig MO is deleted, the agent disconnects the
existing peers, and adds the configured ones. When disconnecting
the existing peers, it makes a call to uv_close on the libuv handle
for the TCP socket. This call is made in a different context than
the libuv context that's processing events. This can lead to a race
condition, where the libuv handle for that socket is marked as
closing, but the libuv thread invokes the pending callbacks for
this handle, resulting in the following assert:

  uv__stream_io: Assertion `!(stream->flags & UV_HANDLE_CLOSING)' failed.

To avoid this, the call to uv_close needs to be deferred to the
libuv processing context. This patch adds a queue to make deferred
close calls, which are handled in the libuv threads async callback.