Add DROP INDEX CONCURRENTLY [IF EXISTS], uses ShareUpdateExclusiveLock

doc/src/sgml/ref/drop_index.sgml

src/backend/catalog/dependency.c

src/backend/catalog/index.c

src/backend/commands/tablecmds.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/tcop/utility.c

src/backend/utils/cache/relcache.c

src/include/catalog/dependency.h

src/include/catalog/index.h

src/include/nodes/parsenodes.h

src/test/regress/expected/create_index.out

src/test/regress/sql/create_index.sql