refactor AsyncProcess

AsyncProcess currently has two patterns of usage, one from HTable flush w/o callback and with reuse, and one from HCM/HTable batch call, with callback and w/o reuse. In the former case (but not the latter), it also does some throttling of actions on initial submit call, limiting the number of outstanding actions per server.
The latter case is relatively straightforward. The former appears to be error prone due to reuse - if, as javadoc claims should be safe, multiple submit calls are performed without waiting for the async part of the previous call to finish, fields like hasError become ambiguous and can be used for the wrong call; callback for success/failure is called based on "original index" of an action in submitted list, but with only one callback supplied to AP in ctor it's not clear to which submit call the index belongs, if several are outstanding.

I was going to add support for HBASE-10070 to AP, and found that it might be difficult to do cleanly.

It would be nice to normalize AP usage patterns; in particular, separate the "global" part (load tracking) from per-submit-call part.
Per-submit part can more conveniently track stuff like initialActions, mapping of indexes and retry information, that is currently passed around the method calls.
I am not sure yet, but maybe sending of the original index to server in "ClientProtos.MultiAction" can also be avoided. Cannot be avoided because the API to server doesn't have one-to-one correspondence between requests and responses in an individual call to multi (retries/rearrangement have nothing to do with it)