Implement master file and new DB header structure in ForestDB

The new architecture requires a separate file that contains DB header info of the main DB file(s). A DB header occupies exactly 4KB page in the current implementation, but header info will be stored as a system document much smaller than 4KB in the master file. It will reduce write amplification further if a write batch size is quite small so that the cost of writing 4KB for DB header is not marginal.

As DB headers are also written in the master file in an append-only manner, we will define a maximum size of the master file (or number of headers), and the oldest DB header system doc will be overwritten by the latest DB header system doc in a circular way.