MB-8606: Parallelizing warmup as a READER task

Improve the efficiency of warmup, by having multiple
threads catering to it, rather than a single thread.

Taking into considering GIO, the warmup task will now
be made a READER_QUEUE task rather than an AUX_QUEUE
task.

The advantage will be with spliting up the keyDump,
LoadingKVPairs and LoadingData state_tasks during
the warmup process into shard level tasks, and have
multiple reader threads cater to each of the sub-
scheduled tasks.

This shouldnt affect any other READER_QUEUE pertinent
task, as traffic is disabled during warmup anyway.

Change-Id: I74fbe3fd97ed34e5af9a1df121ec1988b88be651
Reviewed-on: http://review.couchbase.org/29313
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>