Implement API for setting RM (#419)
* Implement API for setting RM
This is now stored on the server with similar treatment to RRs. The server will only store the specified eventId as the current read marker for a room if the event is ahead in the stream when compared to the existing RM. The exception is when the RM has never been set for this room for this user, in which case the event ID will be stored as the RM without any comparison.
This API also allows for an optional RR event ID to be sent in the same request. This is because it might be the common case for some clients to update the RM at the same time as updating the RR.
See design: https://docs.google.com/document/d/1UWqdS-e1sdwkLDUY0wA4gZyIkRp-ekjsLZ8k6g_Zvso/edit
See server-side PRs: https://github.com/matrix-org/synapse/pull/2120, https://github.com/matrix-org/synapse/pull/2128
