add support for trigerable schedulers to run on multimater mode

- Add a db poller on the triggerable schedulers that polls for completed
buildsets, using the _waiters list
add master's link when running trigerable schedulers in multimaster mode

- create new table to store buildbotURL and add a fk to objectid
- setup configured_buildbotURL, store it or updated in the db
- when trigger build finished if running in multimaster mode get the URL
for buildrequest (comparing objectid from buildrequest_claims and
masters tables)