PR for trac #2621 and trac #2658

 - use builderid as a foreign key instead of using buildername for the table buildrequests
 - addbuilset take a list of buildersID
 - check signatures for the BaseScheduler?'s addBuildsetForXxx methods
 - remove buildername as a data field for buildrequests.
From what I remember, confirmed by looking at several other resource types, we're sticking with singulars in paths.  So `/thing` gets a list of all Things, `/thing/:thingid` gets a single thing, and `/thing/:thingid/part` gets a list of parts associated with Thing number thingid.  In other words, these should all be `/builder` and not `/builders`.