MB-24055: Change default ep-engine ht_size from 3079 to 0
MB-24055: Change default ep-engine ht_size from 3079 to 0

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

Therefore 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 patch). Any existing buckets will keep whatever value they
had, but newly-created buckets can take advantage of this.

After this change, the empty size of an Ephemeral bucket is reduced by
30MB:

    Bucket             RSS (KB)
    Ephemeral(size 3)  57800

  (Note: the default size shouldn't matter from a speed pov - ep-engine
  will automatically resize the HashTable periodicaly (as long as the
  calculated optimum size is greater than the default). Moreover, we
  can happily store more items than the size of the HT, we will simply
  use internal chaining to handle any collisions).

Change-Id: I10f316b24a6a9493e4e424f5904ec09abe33727e
Reviewed-on: http://review.couchbase.org/77264
Tested-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Ajit Yagaty <670a5cadfa9668cb910cc315f6527868cd5ab42a@couchbase.com>
Tested-by: Ajit Yagaty <670a5cadfa9668cb910cc315f6527868cd5ab42a@couchbase.com>

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