Make internal storage format configurable

This also stubs out entity serialization on the client,
where it should never be needed.

DEPLOYMENT: We need to tell the repo to keep writing the old
format to the database for now, since the clients don't support
the new format until the new code has been deployed to all wikis:

  'internalEntitySerializerClass'
      => 'Wikibase\Lib\Serializers\LegacyInternalEntitySerializer'

Bug: 66784
Change-Id: I1a425682b6a594557e3beb4dde685004d4eb84e7
Merge "Make internal storage format configurable"