Move annotation handling to a client?

For #4128 we want drreg to know about all clean calls, yet annotation handler clean calls are inserted during mangling and are thus invisible to clients. The solution there for now is for drreg to look for the DR_NOTE_ANNOTATION label that is added in the app2app phase.

We could move just the clean call insertion to a client in the insertion phase, or try to move the annotation insruction sequence identification: but that may be harder to move.

Alternatively, we could collapse drmgr into the core, and then move the handler clean call to the drmgr insertion phase.

EIther approach would eliminate the drreg special-casing of annotation clean calls, and make them visible to instru2instru and other client actions.