MB-24055: Change default ep-engine ht_size from 3079 to <empty>
MB-24055: Change default ep-engine ht_size from 3079 to <empty>

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 <empty> (i.e. don't
specify a value for ht_size), and instead 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.

Note: A patch attempting to have the same effact was previously merged
      as http://review.couchbase.org/77264, however it was reverted as
      it regressed rebalance time. This was subsequently re-merged
      (same form) as http://review.couchbase.org/#/c/77891/, but this
      triggered a SIGFPE on Linux due to passing a zero value for
      ht_size.
      This version addresses that by omitting 'ht_size' from
      the config string if there isn't a value in the BucketConfig or
      otherwise overridden.

Change-Id: I31a57afa2c24856ceb051c6b0305e7550a6f34b6
Reviewed-on: http://review.couchbase.org/78079
Tested-by: Ajit Yagaty <670a5cadfa9668cb910cc315f6527868cd5ab42a@couchbase.com>
Reviewed-by: Ajit Yagaty <670a5cadfa9668cb910cc315f6527868cd5ab42a@couchbase.com>

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