[4.3] Improve handling of slow websocket clients

[jamesaimonetti]

When a websocket client is having trouble reading messages off its
socket, backpressure will cause the mailbox of the
blackhole_socket_handler to start to fill up with {send_data, _}
tuples from blackhole_data_emitter. This is due to the sending of data
being stuck in prim_inet:send/3.

We add two mechnisms to try to avoid the server being harshly impacted
by slow clients:

1. Reduce the send_timeout blackhole uses to more quickly terminate
   the socket in the event of slow sends
2. Check the mailbox of blackhole_socket_handler from
   blackhole_data_emitter before sending the {send_data, _} tuple, opting
   to shed the load while waiting.