MB-10164: Boosting warmup speed

- Set dbFileRevMapPopulated to true, upon
populating the FileRevMap the first time to avoid
updating the map in every phase of the warmup
process.

- Test results (for the same scenario):

                    ep_warmup_time
2.5.x           :   632.835ms
3.0 master      :   2594.427ms
3.0 with fix    :   263.519ms

Change-Id: Ifbafecb25b175607c25e517e4870a8187f8de7d6
Reviewed-on: http://review.couchbase.org/34585
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>