Trac #2660 [nine] buildrequests data API uses popBooleanFilter for non-booleans
 - added data fields 'branch', 'repository', 'codebase' for buildrequests.
   updated the doc
 - added resultSpec.popStringFilter (popStringFilter == popBooleanFilter)
Trac #2660 [nine] buildrequests data API uses popBooleanFilter for non-booleans
 - This commit implements the 3 dustin's comments:
   - process buildsetid with the eq operator in the DB API
   - implement popStringFilter (only pops an eq filter)
   - add type annotation in master/docs/developer/database.rst
Trac #2660 [nine] buildrequests data API uses popBooleanFilter for non-booleans
 - omit filters 'branch' and 'repository'
 - use popOneFilter instead of popFilter for bsid
Trac #2660 [nine] buildrequests data API uses popBooleanFilter for non-booleans
 - remove fieds: branch, repository, codebase
Trac #2660 [nine] buildrequests data API uses popBooleanFilter for non-booleans
 - remove code in docs/tests about branch/repository/codebase for buildrequests
Merge delanne/buildbot:9/trac2660 (PR #1050)