Allow users to specify reader and writer threads in the UI

Seie MB-36923 for more details.

We should allow users to specify reader and writer thread counts in the UI.

The REST APIs to user are:
GET: pools/default/settings/memcached/global
POST: pools/default/settings/memcached/global 'num_reader_threads=8&num_writer_threads=8'

See this patchset: http://review.couchbase.org/#/c/118320/

We should add an "Advanced Data Service Settings" expando panel on the settings page. It should be below the rebalance settings and above the query settings.

Valid settings for the reader and writer thread values are [0,64]. If 0 it means "use the normal defaults for the box on which Couchbase Server is deployed" - it depends on the number of cores.

Ideally, when set to zero we would show some indication that this is the OOTB default setting. Perhaps we show something like:

Number of reader threads: 0 (use server default value)

Number of writer threads: 0 (use server default value)

Please also include the following text next to the boxes.

Setting higher values for reader and writer threads reduces latency in persisting items to and reading items from disk and increases throughput for synchronous write operations. It can result in greater write amplification and server CPU utilization. Consult Couchbase documentation and use care in setting these values.