WIP: Fast transaction encode & decode

Current transaction use a bufferlist tbl to encode all actions and their data, so if object appears many times, it will be encoded many times. And when FileStore do this transaction, it must decode all actions and data from the tbl.

This path follows BP: osd: update_Transaction_encoding, 

The transaction use two maps to keep all colls and objects appeared in the transaction, and each coll and object only need to be encoded once. On the other hand, FileStore don't need to decode actions, colls and objects anymore.
would it be simpler to make class iterator pure virtual and have two child classes, instead of explictitly wrapping around each impl method here?  (don't really care)