Use the shard serial feature of ExecutorPool on writer tasks

This results in better thread usage since threads will never
get stuck trying to acquire the shard lock

The shard lock is still needed for the flushAllDelete operation
which should be rare

Change-Id: I4e1ee2c28bd15c4066e40b1feb34c1eff9ea6cfb
Reviewed-on: http://review.couchbase.org/36420
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>