[#245] Resolve "use socket pool for all signald messages"

Closes #245 (closed)

i promise to write something more detailed soon! but... basic idea is:

• make the socket pool a subcomponent of the app module
• always use the socket pool for all signald writes
• introduce a callbacks registry to handle signald logic with a request/response cycle (ie: trust, register and version commands)
• make necessary changes to calling clients and test harness to accomodate these changes

cleanup i want to circle back to:

• write some integration tests for register and trust
• cleanup the signal directory a bit (try to not initialize socket in signal.run, rename signal.signal to signal.index etc...

cc @fdbk for post-merge CR!