pgsql: Add option SKIP_LOCKED to VACUUM and ANALYZE

Add option SKIP_LOCKED to VACUUM and ANALYZE

When specified, this option allows VACUUM to skip the work on a relation
if there is a conflicting lock on it when trying to open it at the
beginning of its processing.

Similarly to autovacuum, this comes with a couple of limitations while
the relation is processed which can cause the process to still block:
- when opening the relation indexes.
- when acquiring row samples for table inheritance trees, partition trees
or certain types of foreign tables, and that a lock is taken on some
leaves of such trees.

Author: Nathan Bossart
Reviewed-by: Michael Paquier, Andres Freund, Masahiko Sawada
Discussion: https://postgr.es/m/9EF7EBE4-720D-4CF1-9D0E-4403D7E92990@amazon.com
Discussion: https://postgr.es/m/20171201160907.27110.74730@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/803b1301e8c9aac478abeec62824a5d09664ffff

Modified Files
--------------
doc/src/sgml/ref/analyze.sgml | 19 +++
doc/src/sgml/ref/vacuum.sgml | 21 +++
src/backend/commands/vacuum.c | 26 +++-
src/backend/parser/gram.y | 12 ++
src/include/nodes/parsenodes.h | 3 +-
src/test/isolation/expected/vacuum-skip-locked.out | 171 +++++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/specs/vacuum-skip-locked.spec | 59 +++++++
src/test/regress/expected/vacuum.out | 16 +-
src/test/regress/sql/vacuum.sql | 12 +-
10 files changed, 333 insertions(+), 7 deletions(-)