Bug 1821360

This was found when analysing failures in the early prototypes of the async IO patchset.

Initial condition:

A DB is removed from OVSDB due to an admin request or a raft HA decision

This triggers notifications for monitor cancels of all clients which are subscribed to the database. These notifications are enqueued to be sent to clients which takes a finite amount of time. 

Any client which will issue a transaction to the database during this time window will receive a "syntax error" JSON reply.

This will be extremely difficult to fix without major API additions because there is no mandatory flush and there is no "upper level" means of triggering a json rpc "echo" and waiting for an echo reply to ensure that anything on the wire between the server and the client has been flushed.

This is likely to be an issue ONLY at scale when there are a lot of pending requests and a lot of pending notifications to transmit.

It is somewhat mitigated by ovsdb connection being effectively half-duplex and it not invoking jsonrpc session receive if there is pending transmit. While this mitigates it, it does not fix it. If ovsdb is optimized for throughput in any way, this is likely to become easier to reproduce.