Finish-up AMv2 Design/List of Tenets/Specification of operation

AMv2 is missing specification. There are too many grey-areas still. Also missing are a concise listing of the tenets of AMv2 operation. Here are some examples:

• HBASE-19529 "Handle null states in AM": Asks how we should treat null state in hbase:meta. What does it 'mean'. We seem to treat it differently dependent on context. Needs clarification. Apache9 recently asked similar about the meaning of OFFLINE.
• Logging needs to have a particular form to help trace Procedure progress; needs a write-up.

Lets fill in items to address in this umbrella issue. Can address in subissues and produce specification doc too. We have the below but these are mostly (incomplete) description for devs on pv2 and amv2; the specification is missing:

http://hbase.apache.org/book.html#pv2
http://hbase.apache.org/book.html#amv2

(Other areas include addressing what is up w/ rollback – when, how much, and when it is not appropriate – as well as recommendation on Procedures coarseness, locking – is it ok to lock table in alter table procedure for the life of the procedure? – and so on).