Rename TransactionChain functions

Peter Eisentraut <peter_e@gmx.net>

Commit: 04700b685f31508036456bea4d92533e5ceee9d6
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2018-03-16T17:18:06Z
Releases: 11.0
Rename TransactionChain functions

We call this thing a "transaction block" everywhere except in a few
functions, where it is mysteriously called a "transaction chain".  In
the SQL standard, a transaction chain is something different.  So rename
these functions to match the common terminology.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>

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