MB-8679 Add pendingVbs queue to bgfetcher

This queue allows bgfetcher to visit only those vbuckets
in the given shard that have pending bgfetch requests.
This optmizes scheduling bgfetch requests in more efficient
fifo way.

Rebalance should benefit the above enhancement most, and initial
test shows below performance increase.

<Reb-1-out-3-buckets>

                2.1.0-718-rel ||  2.1.1-with-this-fix

Time hrs        4.48          ||  3.83

Rebalance
Time (sec)      3095          ||  2578

Sets (90-95-99)
(ms)            11 – 17 – 53  ||  6 – 9 – 18

Gets (90-95-99)
(ms)            13 – 20 – 208 ||  9 – 14 – 164

Ops (avg)       2.40k         ||  1.48k

Change-Id: Iadbbcb069701da947aac2a8cbc0d8b30762a13a4
Reviewed-on: http://review.couchbase.org/27178
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>