Rename worker_init as worker_create to align with other code.
Rename worker_t as socket_worker_t.
Rename worker_pool_t as socket_worker_pool_t.

Somewhat debatable because the disk writers/workers are
also here, but they hang off of socket workers.
Rename *worker*[hc] as socket_
Rename the socket worker APIs to match with the types and filenames.
Move all the global state to a single struct.

(This is a good step towards having *no* global state.)
The control 'word' is really also global state.
MSG_NOSIGNAL only used in socket_worker.c
These only used in relay.c
Add per-worker locks.
Add option to control the decoding of messages.
Release the process status buffer when shutting down.