During decommission, the storage_service::unbootstrap() needs to
initiate a batchlog replay operation. To sync the replay operation
initiated by the timer in batchlog_manager and storage_service, a
semaphore is introduced. To simplify the semaphore locking, the
management code now always runs on shard zero, but the real work is
distruted to all shards.
Somewhat simplifies version of the Origin code, since from what I
can see, there is less need for us to do explicit query sends in
the BLM itself, instead we can just go through storage_proxy.
I could be wrong though.
