Allow "transaction.search" to be called on an object type

Summary:
Ref T13631. This supports a more robust version of "poll for updates by using dateModified window queries" that uses transactions as a logical clock.

This is particularly relevant for commits, since they don't have a "dateModified" at time of writing.

Test Plan:

• Queried for transactions by type and object.
• Issued various invalid transaction queries, got appropriate errors.

Maniphest Tasks: T13631

Differential Revision: https://secure.phabricator.com/D21601