Fix memory usage for verifiers. Events to verify were being loaded
from the db into an in-memory fifo queue to spool to worker processes.
This was not being limited, resulting in a large amount of memory
being used if events were read from the DB faster than they were
being processed. This change pauses the loading of events if the
in-memory queue grows larger than specified batchsize.
Also, verifier child processes were not handling signals (like SIGTERM)
properly, resulting in them not shutting down properly.
Added proper signal handling.
Change-Id: Ife25ca07398acf111f4388071b5f2e4eafeecb05
f5a03f1afe