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

src/backend/parser/gram.y

@@ -10538,6 +10538,8 @@ vacuum_option_elem:
 				{
 					if (strcmp($1, "disable_page_skipping") == 0)
 						$$ = VACOPT_DISABLE_PAGE_SKIPPING;
+					else if (strcmp($1, "skip_locked") == 0)
+						$$ = VACOPT_SKIP_LOCKED;
 					else
 						ereport(ERROR,
 								(errcode(ERRCODE_SYNTAX_ERROR),
@@ -10571,6 +10573,16 @@ analyze_option_list:
 
 analyze_option_elem:
 			VERBOSE				{ $$ = VACOPT_VERBOSE; }
+			| IDENT
+				{
+					if (strcmp($1, "skip_locked") == 0)
+						$$ = VACOPT_SKIP_LOCKED;
+					else
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("unrecognized ANALYZE option \"%s\"", $1),
+									 parser_errposition(@1)));
+				}
 		;
 
 analyze_keyword: