Account for worker forking in FieldLogWriter startup.

fork(2) copies only a single thread from its parent process, which is
the thread that called fork(2).  All other parent threads do not exist
in the child.  Ruby 1.9.x maps Ruby threads to system threads, so we
need to account for this.  (Previously, we weren't accounting for this,
which meant that the field log writer wasn't responding to messages at
all.)

The accounting method is pretty simple: if we're in a known
workers-will-fork situation, then we start the field log writer
supervisor (and, by extension, the writer) for each worker process (not
the master).  If we're not in a situation like that, we assume starting
the writer supervisor at startup is the right thing to do.  AFAICT, this
covers all NUBIC use cases.

Fixes #4141.