Make ExternalStore wrap ExternalStoreFactory and create access class

Make ExternalStore wrap ExternalStoreFactory and create access class

* Inject settings and global instances as dependencies to the
  ExternalStoreMedium instances. This includes the local wiki
  domain, so that wfWikiId() calls are not scattered around.
* Create ExternalStoreAccess service for read/write logic.
* Deprecate the ExternalStore wrapper methods.
* Add some exception cases for bogus store URLs are used instead
  of just giving PHP warnings and failing later.
* Make moveToExternal.php require the type/protocol to decide
  which ExternalStoreMedium to use instead of assuming "DB".
* Convert logging calls to use LoggerInterface.

Change-Id: I40c3b5534fc8a31116c4c5eb64ee6e4903a6197a