MB-12562: Avoid terminating all writer threads

With incremental writer thread scheduling, we
need to make sure that all writer threads aren't
deleted before deleting all buckets.

Scenario
- maxWriters is 4
- create bucket 1, numWriters=1
- create bucket 2, numWriters=2
- create bucket 3, numWriters=3
- create bucket 4, numWriters=4
- create bucket 5, numWriters=4
- delete bucket 5, numWriters=3 =>with change, 4
- delete bucket 4, numWriters=2 =>with change, 3
- delete bucket 3, numWriters=1 =>with change, 2
- delete bucket 2, numWriters=0 =>with change, 1
- delete bucket 1, numWriters=0 =>with change, 0

Change-Id: Ib9d45f7acb9177924612547538aa74ca9dd49c20
Reviewed-on: http://review.couchbase.org/42880
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>