Fix support for exclusion constraints. (#25)

* Fix support for exclusion constraints.

* Include ALTER TABLE ADD constraint timing to the summary.

* Improve a summary label.

* Bug fix: recognize index supporting constraints.

The "constraint" boolean arg is about our intend to either create the index
or the constraint (that maybe attached to the index). Our decision making
about the concurrent building of indexes should apply to building an index
that is baking a constraint that's neither PK or UNIQUE, we can't re-use our
"constraint" boolean for that decision making.

* Refactoring: fix a function's name.

Using the start_ prefix in the function name would imply async behavior
where copydb_create_table_indexes actually waits until all the indexes have
been built before returning. Change its name.

* Refactor code to make it easier to read.