MDL-42071 caching Support for clustered memcache 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 memcache 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_MEMCACHE_TESTSERVERS with
multiple (return delimited) servers.
submodules