MB-10437: prefer oldest waiter in concurrency_throttle

Old behavior could choose same pid(s) over and over again. Which
caused particular problem for initial replication under slight udpates
load. I.e. it would repeatedly pick already mostly up-to-date vbuckets
over vbuckets that still need to be replicated. Which would cause very
short batches which in turn would cause very slow rebalance.

By picking oldest waiter (modulo system clock jumps which should be
infrequent) we avoid this problem. It should also help at least
somewhat for the case of ongoing replication under somewhat heavy but
not too heavy load, due to better avoidance of too short replication
batches.

Change-Id: Id9fc64ee08b77f6a93760f8c4de237f14fe5ebd9
Reviewed-on: http://review.couchbase.org/38368
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>