FileStore: ping TPHandle after each operation in _do_transactions

Each completed operation in the transaction proves thread
liveness, a stuck thread should still trigger the timeouts.

Fixes: #3928
Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
HeartbeatMap: add heartbeat_debug_disable_suicide for testing

Signed-off-by: Samuel Just <sam.just@inktank.com>