Locks for Param, ParamSet, & ParamSelector (used by Stage, Pipeline, DistributionMaker)

[jllanfranchi]

• registering parent sets one's lock to be parent's lock object
• disconnecting parent grabs new lock
• @synchronized decorator so we don't have to copy-paste and add lots of explicit mutex code
  • Shockingly, others have had the same idea; see e.g. https://github.com/GrahamDumpleton/wrapt/blob/develop/blog/07-the-missing-synchronized-decorator.md and http://stackoverflow.com/questions/29402606/possible-to-create-a-synchronized-decorator-thats-aware-of-a-methods-object

[LeanderFischer]

@synchronized looks like a cool thing, but we've been doing well without, so maybe we don't need it..