Fix connection failure handling in MemcachedCacheSource

The previous test did not work at all, as `get` does not error out
if the connection fails, but rather returns that the key is not
available.

`set` will return false if the item could not be saved. In addition
\Memcached::OPT_REMOVE_DAFILED_SERVERS is set, which will cause the
broken memcached servers to be ignored for the rest of the execution.
The keys intended for the broken server will transparently be mapped
to other memcached servers.
Use \Memcached::OPT_BINARY_PROTOCOL in production mode
Remove superfluous serializing of master record in MemcachedCacheSource
Use Memcached::OPT_PREFIX_KEY instead of manually prefixing each key
Write memcached cache master atomically
Update master in O(1) instead of O(n)
Get rid of master record in MemcachedCacheSource
Add WCF_UUID option
Prefix Memcached keys with WCF_UUID instead of hash of WCF_DIR
Merge branch 'memcached'
Merge branch 'master' of github.com:WoltLab/WCF