MessageCache: use APC for local caching, rather than files

MessageCache: use APC for local caching, rather than files

In addition to eliminating disk IO in a hot path, using APC spares us from
having to serialize and unserialize cache arrays. Since we're not serializing,
though, we don't have a string representation to hash, so use a random string
instead. (The code already treats the association of hash string to cache as
purely symbolic, so this is not problematic.)

Whereas the hash was previously stored as the first 32 bytes of each cache
file, we now store it as an array key instead (like VERSION and EXPIRY were
already). Because this changes the structure of cached data, we have to bump
MSG_CACHE_VERSION.

While we're here, make MessageCache::getLocalCache() and
MessageCache::saveToLocal() protected, make their signatures more
consistent with other methods in this class. While they were (implicitly)
public before, there are absolutely no external callers in Core or
extensions[0][1], so we can skip the standard deprecation process.

[0]: https://github.com/search?q=%40wikimedia+getlocalcache&type=Code&utf8=%E2%9C%93
[1]: https://github.com/search?utf8=%E2%9C%93&q=%40wikimedia+savetolocal&type=Code

Change-Id: I020617d2df2a8f0f243b85f3383dc7b16f15aaad