Add 'max_connections' setting to UI / REST.

Edit: Updated to new VERIFY/RELOAD messages as discussed in parent (MB-11066)

We want to be able to change the maximum number of connections in memcached dynamically. We have MB-11066 (parent of this) to add the support in memcached to allow this, but we also need an interface for the user to do this.

I'll leave it up to ns_server to determine the best way to show this in the UI (but one suggestion is to add it to the "internalSettings" window).

As discussed with Alk and agreed with Trond, going with an alternative mechanism - will add two new binary protocol commands - CONFIG_VERIFY and CONFIG_RELOAD.

• CONFIG_VERIFY passes a proposed new config to memcached in the body, which memcached will verify and return either SUCCESS or EINVAL if the config is not valid (for example attempting to change settings which cannot be changed dynamically).
• CONFIG_RELOAD instructs memcached to reload the memcached.json file from disk. Any modified parameters which are permitted to be changed dynamically will be updated.

See http://review.couchbase.org/#/c/40751/ for memcached changes to implement this.