Turn ParserCache into a service, deprecate $parserMemc

Turn ParserCache into a service, deprecate $parserMemc

ParserCache is already a singleton, making it a good candidate for a
service. $parserMemc is an odd global (it lacks the "wg" prefix) and is
ripe for deprecation.

The following are now deprecated:
* $parserMemc global
* ParserCache::singleton()
* wfGetParserCacheStorage()

A ParserCache::getCacheStorage() method was added for cases where direct
access to the underlying BagOStuff object is necessary.

Usage of $parserMemc will emit deprecation warnings through the
DeprecatedGlobal class mechanism. All usage in core was migrated.

Also take this opportunity to inject the $wgCacheEpoch global value into
ParserCache. This will require an update to the FlaggedRevs extension.

Change-Id: I2ac7afff0d8522214329248c3d1cdccd0f72bbd4