Add UUID to process identity.

In environments with deterministic hostnames and PIDs (i.e. Heroku), the
current system for constructing a process's identity string produces
collisions (e.g. a new process can assume the identity of one that has
died). In order to address this, we add a UUID to the identity string.

This has been tested with a local sidekiq server (I ran some jobs and
verified that signaling from the UI works properly).

Issue #2071.