Add verification of handoff heartbeat to verify_handoff test

Add testing of the handoff heartbeat change from the following pull
request: https://github.com/basho/riak_core/pull/560. Add an intercept
module for the riak_core_handoff_sender module to introduce artificial
delay on item visitation during a handoff fold. This delay along with
the changes to the verify_handoff test induces test failure when run
without the heartbeat change. The handoff_receive_timeout is exceeded,
handoff stalls, and the test eventually fails due to timeout. The test
succeeds when run with the heartbeat change.
Merge pull request #681 from basho/feature/verify-handoff-heartbeat

Add verification of handoff heartbeat to verify_handoff test