MDL-42071 caching Support for clustered memcached caching.

For stores where there is a very high rate of sets compared to gets, it
is beneficial to retrieve from the local server, skipping the network
overhead, at the expense of having to set many severs when a key is set.

This patch adds a memcached option to enable clustering. When on, only
one "server" is allowed, which will be where fetches are from, while
sets/updates/deletes/purges will occur to the all the servers in the
"set server" list.

To run unit tests, define TEST_CACHESTORE_MEMCACHED_TESTSERVERS with
multiple (return delimited) servers.