pgsql: Fix VACUUM's reporting of dead-tuple counts to the stats collect

Fix VACUUM's reporting of dead-tuple counts to the stats collector.

Historically, VACUUM has just reported its new_rel_tuples estimate
(the same thing it puts into pg_class.reltuples) to the stats collector.
That number counts both live and dead-but-not-yet-reclaimable tuples.
This behavior may once have been right, but modern versions of the
pgstats code track live and dead tuple counts separately, so putting
the total into n_live_tuples and zero into n_dead_tuples is surely
pretty bogus.  Fix it to report live and dead tuple counts separately.

This doesn't really do much for situations where updating transactions
commit concurrently with a VACUUM scan (possibly causing double-counting or
omission of the tuples they add or delete); but it's clearly an improvement
over what we were doing before.

Hari Babu, reviewed by Amit Kapila

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/115f414124e71749d2d8f512e469ca63bc2166e5

Modified Files
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src/backend/commands/vacuumlazy.c |   13 +++++++++++--
src/backend/postmaster/pgstat.c   |   11 ++++++-----
src/include/pgstat.h              |    5 +++--
3 files changed, 20 insertions(+), 9 deletions(-)