postpone write parameter

[eulerto]

Currently, the plugin outputs JSON in pieces (got data and sent it). The receiver has to maintain the state to know if a complete JSON object has already arrived then it can process it.

At least for the SQL interface POV, it would be a good idea to support buffering JSON object until its end. It would avoid maintain the state in the receiver. The drawback is long transactions would consume a lot of server memory.

PS> unfortunately, pg_recvlogical can't take advantage of this option because you can only provide parameters at startup. Of course, you could choose a convenient default for all your streaming.

[eulerto]

Done in commit 7c9b483