Track statistics for streaming of in-progress transactions

Commit 464824323e has added the support of the streaming of
in-progress transactions into the built-in logical replication. The
attached patch adds the statistics about transactions streamed to the
decoding output plugin from ReorderBuffer. Users can query the
pg_stat_replication_slots view to check these stats and call
pg_stat_reset_replication_slot to reset the stats of a particular
slot. Users can pass NULL in pg_stat_reset_replication_slot to reset
stats of all the slots.

Commit 9868167500 has added the basic infrastructure to capture the
stats of slot and this commit extends the statistics collector to
track additional information about slots.

This patch was originally written by Ajin Cherian [1]. I have fixed
bugs and modified some comments in the code.

Thoughts?

[1] - https://www.postgresql.org/message-id/CAFPTHDZ8RnOovefzB%2BOMoRxLSD404WRLqWBUHe6bWqM5ew1bNA%40mail.gmail.com

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With Regards,
Amit Kapila