Mark metadata_update RPC call deprecated

Log a deprecation warning if the metadata_update call is used,
in theory nothing uses this now, but it's possible folks could
try running the engine with a slightly older API version, or
have out-of-tree stuff using this RPC call (although I'm not
aware of any..), so maintain it for a transitional period (TBC,
maybe remove it after J2 has passed, or do we need to leave it
until K?)

Note the intention is to remove the metadata_update RPC call, but
maintain the metadata_update plugin interface (which is now only
used when it's necessary to do an internal refresh of the metadata
all external metadata update should happen via the resource_signal
interface, so it's compatible with the native API.)

Change-Id: Idf79389bc4250f91959cc13ae5fe7513aff6f291
Merge "Mark metadata_update RPC call deprecated"