Like BASE_BACKUP, CREATE_REPLICATION_SLOT has historically used a
hard-coded syntax. To improve future extensibility, adopt a flexible
options syntax here, too.
In the new syntax, instead of three mutually exclusive options
EXPORT_SNAPSHOT, USE_SNAPSHOT, and NOEXPORT_SNAPSHOT, there is now a single
SNAPSHOT option with three possible values: 'export', 'use', and 'nothing'.
This commit does not remove support for the old syntax. It just adds
the new one as an additional option, makes pg_receivewal,
pg_recvlogical, and walreceiver processes use it.
Patch by me, reviewed by Fabien Coelho, Sergei Kornilov, and
Fujii Masao.
Discussion: http://postgr.es/m/CA+TgmobAczXDRO_Gr2euo_TxgzaH1JxbNxvFx=HYvBinefNH8Q@mail.gmail.com
Discussion: http://postgr.es/m/CA+TgmoZGwR=ZVWFeecncubEyPdwghnvfkkdBe9BLccLSiqdf9Q@mail.gmail.com
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