review: create if not exists

Hello,

this patch introduces IF NOT EXISTS clause to CREATE (AGGREGATE | CAST
|COLATION |DOMAIN |ROLE |SEQUENCE |SEARCH CONFIGURATION |SEARCH DICTIONARY
| SEARCH PARSE | SEARCH TEMPLATE | TYPE). Implemented feature was discussed
more times in spring and summer, and this patch was ready for commit in
summer. Fabrízio reflected all reviewers comments. I primary checked a
consistency of related patch.

* all new functionality (syntax) is well documented
* there are no any compilation warning related to implemented feature
* all new functionality is tested in regress tests
* new code fully respects PostgreSQL coding standards
* new code is almost mechanical - and enough documented
* new functionality has no impact on performance
* new functionality doesn't need a changes in pg_dump

This patch is ready for commit

Regards

Pavel Stehule