… thread

On iOS (_SAL_TIME_H) socket.io has no asio network thread — all I/O runs on
the single SocketQueueKpoll loop, which also processes inbound frames
(onSocketMessage → handle_read_frame). A websocketpp connection is not
internally locked: every operation on it must run on that one thread.
client_impl::send_impl() called m_client.send(m_con, ...) inline on the
caller's thread. emit*() runs on app threads and the ping runs on the timer
thread, so a write could execute concurrently with SocketQueueKpoll inside
handle_read_frame on the same connection. That race corrupts the connection's
shared read/write state and crashes on the read side in
sio::packet_manager::put_payload (EXC_BAD_ACCESS, faulting pointer holding
inbound-payload bytes).
The earlier teardown fix only serialized close/sync_close; the write path was
still inline. All writers funnel through send_impl (only m_client.send call
site; emit, ping, connect and on_encode all route through it), so hop the
write onto the socket-queue thread there — inline when already on it
(handshake sends from on_open), Post() otherwise. Inbound handling, writes and
close now all run on the one thread.
Reuses the existing SAL::MSocketQueueManager Post/IsOnQueueThread hooks; no
ABI/layout change. Non-_SAL_TIME_H (asio) path unchanged — its io_service
strand already serializes.

Change-Id: Ie3a8260714ca008fc810436b10f8b9d325c5b3b1