SingleBranchScheduler: add createAbsoluteSourceStamps config option

This option affects multi-codebase setups. When this option is enabled,
it uses the last revision seen for each codebase without a change instead
of using the latest revision.
SingleBranchScheduler: add description and note for createAbsoluteSourceStamps argument.
SingleBranchScheduler: only access database if createAbsoluteSourceStamps enabled
SingleBranchScheduler: remove shouldntBeSet argument and return None in defer
Merge branch 'sbs-abs-source-stamps' of git://github.com/thedylman/buildbot