This PR does a few things.
It fixes a shutdown-order flaw in the chainfreezer. Previously, the chain-freezer would shutdown the freezer backend first, and then signal for the loop to exit. This can lead to a scenario where the freezer tries to fsync closed files, which is an error-conditon that could lead to exit via log.Crit.
It also makes the printout more detailed when truncating 'dangling' items, by showing the exact number instead of approximate MB.
This PR also adds calls to fsync files before closing them, and also makes the `db inspect` command slightly more robust.
This PR fixes an issue which might result in data lost in freezer.
Whenever mutation happens in freezer, all data will be written into head data file
and it will be rotated with a new one in case the size of file reaches the threshold.
Theoretically, the rotated old data file should be fsync'd to prevent data loss.
In freezer.Sync function, we only fsync: (1) index file (2) meta file and (3) head
data file. So this PR forcibly fsync the head data file if mutation happens in the
boundary of data file.
Previously freezer has only been used for storing ancient chain data, while obviously it can be used more. This PR unties the chain data and freezer, keep the minimal freezer structure and move all other logic (like incrementally freezing block data) into a separate structure called ChainFreezer.
This PR also extends the database interface by adding a new ancient store function AncientDatadir which can return the root directory of ancient store. The ancient root directory can be used when we want to open some other ancient-stores (e.g. reverse diff freezer).
