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 patch 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.
> Yes, I want to simplify the alignment calculations. When the data_bl is at the front, I can just use largest_data_off_in_tbl without any
> additional calculation.

keep in mind that the ENCODE_START (if i remember correctly) will put it a 
__u32 and two __u8's