Started on `chkrec_worker` behaviour.

Intended to extend the `gen_server` behaviour to provide automated
state recovery (and message retransmission) in the case of a single
process failure.

Defined the same callbacks as done so by `gen_server`, but actually
need to defer to them in last pattern match case.
Updated the specs of all the `chkrec_worker` functions.

Need to support any possible return value that the `gen_server`
behaviour allows according to the callback definitions we made.
Made functions defer to the callback module.

Lots of boring code to handle the various cases.
Renamed `chkrec_worker` module to `chkrec_server`.

Make the fact that we are extending the `gen_server` behaviour
slightly more obvious.
Defined options to `chkrec_server:init/1`.

A backup should occur when either of the conditions are met:
  (1) `TimeFreq` milliseconds have passed
  (2) `ReceiveThresh` messages were received