pillowfight.cc: allow gracefully terminate the loop

The benefit of this changes is that pillowfight application won't mess
valgrdind output on premature termination because signal handler will
call destructor of ThreadContext

Change-Id: I3629d1df44f09f1c51ee1bb0da6897cacb81a67e
Reviewed-on: http://review.couchbase.org/25086
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>