pgsql: Make DROP DATABASE command generate less WAL records.

Make DROP DATABASE command generate less WAL records.

Previously DROP DATABASE generated as many XLOG_DBASE_DROP WAL records
as the number of tablespaces that the database to drop uses. This caused
the scans of shared_buffers as many times as the number of the tablespaces
during recovery because WAL replay of one XLOG_DBASE_DROP record needs
that full scan. This could make the recovery time longer especially
when shared_buffers is large.

This commit changes DROP DATABASE so that it generates only one
XLOG_DBASE_DROP record, and registers the information of all the tablespaces
into it. Then, WAL replay of XLOG_DBASE_DROP record needs full scan of
shared_buffers only once, and which may improve the recovery performance.

Author: Fujii Masao
Reviewed-by: Kirk Jamison, Simon Riggs
Discussion: https://postgr.es/m/CAHGQGwF8YwNH0ZaL+2wjZPkj+ji9UhC+Z4ScnG97WKtVY5L9iw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e6d8069522c8bde8239dd1fedfb4984efa4b3a1a

Modified Files
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src/backend/access/rmgrdesc/dbasedesc.c |  7 +++-
src/backend/commands/dbcommands.c       | 67 +++++++++++++++++++++++----------
src/include/commands/dbcommands_xlog.h  |  5 ++-
3 files changed, 56 insertions(+), 23 deletions(-)