[COMMITTERS] pgsql: Avoid having vacuum set reltuples to 0 on non-empty relationsin

Avoid having vacuum set reltuples to 0 on non-empty relations in the
presence of page pins, which leads to serious estimation errors in the
planner.  This particularly affects small heavily-accessed tables,
especially where locking (e.g. from FK constraints) forces frequent
vacuums for mxid cleanup.

Fix by keeping separate track of pages whose live tuples were actually
counted vs. pages that were only scanned for freezing purposes.  Thus,
reltuples can only be set to 0 if all pages of the relation were
actually counted.

Backpatch to all supported versions.

Per bug #14057 from Nicolas Baccelli, analyzed by me.

Discussion: https://postgr.es/m/20160331103739.8956.94469@wrigleys.postgresql.org

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1914c5ea7daaaaba4420f65c991256af5d4a9813

Modified Files
--------------
src/backend/commands/vacuumlazy.c                | 15 ++++--
src/test/isolation/expected/vacuum-reltuples.out | 62 ++++++++++++++++++++++++
src/test/isolation/isolation_schedule            |  1 +
src/test/isolation/specs/vacuum-reltuples.spec   | 45 +++++++++++++++++
4 files changed, 119 insertions(+), 4 deletions(-)