[pv2] AssignmentProcedure is too coarse grained

Comes of work on HBASE-20100 but in particular, in feedback from Apache9 https://mail.google.com/mail/u/0/#inbox/161d8e41054be406

The AP is too coarse-grained. There is precheck+start, then transform state is edit meta setting state to OPENING and then dispatch (rpc).... Finish is edit of meta and setting internal state. The edit of meta should be distinct step at least.

Would save on duplicated ops – e.g. re-editing hbase:meta and dispatching another RPC – if we fail going into finishing. Apache9 brings up our perhaps masking other state change hiccups when steps are so coarse-grained.

Do same for unassignprocedure.