fix mixture of sync/async sockets in IOPubThread

[minrk]

all sockets are explicitly sync until/except we are in the coroutines that will await them, whereas there was an ambiguous mixture of sync and async sockets before

• consistent behavior of send for child pipe and main process sockets
• avoids unsafe assumption that send is greedy on async sockets
• avoids potential issues creating async objects in one thread, then using them in another in a different event loop
• always creates/uses the right types, regardless of input socket
• address some typing lint

found while working on https://github.com/ipython/ipyparallel/issues/895

[minrk]

ok, actually all correct now, I think