MB-24055: Change default ep-engine ht_size from 3079 to 0 (take 2)
MB-24055: Change default ep-engine ht_size from 3079 to 0 (take 2)

Note: This was previously merged as http://review.couchbase.org/77264,
      however it was reverted as it regressed rebalance
      time. Re-submitting again to give ep-engine control of the
      default ht_size; however with the same default value (3079) as
      ns_server had. As such this is logically a no-op.

The bucket config option ht_size controls the initial and minimum
vBucket HashTable size for ep buckets; measured in terms of HashTable
slots. Given we normally have 1024 vBuckets, this creates 3.1M slots
in the HashTable.

For small buckets this adds a lot of unnecessary overhead -
particulary comparing Ephemeral to memcache buckets, where their empty
size (single node) is:

    Bucket             RSS (KB)
    <none>             40096
    memcache           40260
    Ephemeral          87504

To allow a more sensible default to be selected, change the default
ht_size from a fixed number in ns_server to zero, which will let
ep_engine pick it's own default (see linked ep-engine patchs). Any
existing buckets will keep whatever value they had, but newly-created
buckets can take advantage of this.

Change-Id: I99ecc4639df0697b1a91ba6d13cecbfac19f49a5
Reviewed-on: http://review.couchbase.org/77891
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server