Rewrite the ad-hoc parser for Postgres Archive TOC items. (#469)

* Rewrite the ad-hoc parser for Postgres Archive TOC items.

This time use a tokenizer and hand-roll a grammar/parser on-top of it,
expecting tokens in the right order. We have a pretty simple grammar at the
moment, because we don't extend the support to the ACL and COMMENT composite
types. We might want to do that later.

* Reformat parsing of ACL and COMMENTS too.

* Free allocated memory for parsing pg_restore archive catalogs.

* Fix rebase.

* Protect against NULL pointer dereferencing.

* Fix string size compute bug (include 2 bytes for prefix).