pgsql: Fix O(N^2) stat() calls when recycling WAL segments

Fix O(N^2) stat() calls when recycling WAL segments

The counter tracking the last segment number recycled was getting
initialized when recycling one single segment, while it should be used
across a full cycle of segments recycled to prevent useless checks
related to entries already recycled.

This performance issue has been introduced by b2a5545, and it was first
implemented in 61b86142.

No backpatch is done per the lack of field complaints.

Reported-by: Andres Freund, Thomas Munro
Author: Michael Paquier
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/20170621211016.eln6cxxp3jrv7m4m@alap3.anarazel.de
Discussion: https://postgr.es/m/CA+hUKG+DRiF9z1_MU4fWq+RfJMxP7zjoptfcmuCFPeO4JM2iVg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5ae1572993ae8bf1f6c33a933915c07cc9bc0add

Modified Files
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src/backend/access/transam/xlog.c | 61 ++++++++++++++++++++-------------------
1 file changed, 31 insertions(+), 30 deletions(-)