Remove :timestamp column type

The `:timestamp` type for columns is unused. All database adapters treat
them as the same database type. All code in `ActiveRecord` which changes
its behavior based on the column's type acts the same in both cases.
However, when the type is passed to code that checks for the `:datetime`
type, but not `:timestamp` (such as XML serialization), the result is
unexpected behavior.

Existing schema definitions will continue to work, and the `timestamp`
type is transparently aliased to `datetime`.
Merge pull request #15184 from sgrif/sg-remove-timestamp-type

Remove :timestamp column type
Revert "Rewrite journey routes formatter for performance"

This reverts commit 5c224de9e110763ec7a0f01f5b604bcf81f40bfb.

Conflicts:
	actionpack/lib/action_dispatch/journey/visitors.rb

5c224de9e110763ec7a0f01f5b604bcf81f40bfb introduced a bug in the
formatter.  This commit includes a regression test.