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Support the new monitor method monitor_cond_since so that a client can request monitoring start from a specific point instead of always from beginning. This will reduce the cost at scenarios when server is restarted/failed-over but client still has all existing data. In these scenarios only new changes (and in most cases no change) needed to be transfered to client. When ovsdb-server restarted, history transactions are read from disk file; when ovsdb-server failed over, history transactions exists already in the memory of the new server. There are situations that the requested transaction may not be found. For example, a transaction that is too old and has been discarded from the maintained history list in memory, or the transactions on disk has been compacted during ovsdb compaction. In those situations the server fall backs to transfer all data start from begining. For more details of the protocol change, see Documentation/ref/ovsdb-server.7.rst. This change includes both server side and ovsdb-client side changes with the new protocol. IDLs using this capability will be added in future patches. Now the feature takes effect only for cluster mode of ovsdb-server, because cluster mode is the only mode that supports unique transcation uuid today. For other modes, the monitor_cond_since always fall back to transfer all data with found = false. Support for those modes can be added in the future. Signed-off-by: Han Zhou <hzhou8@ebay.com> Signed-off-by: Ben Pfaff <blp@ovn.org>