When during a checkpoint more than one file is created the entries position in
the index is not right since it's not reset at every new index.
Fix it and add related tests.
* export: exports the newest data checkpoint. It forces a checkpoint before
exporting (currently no wals are exported)
* import: cleans up etcd, creates a new datasnaphot from the provided import stream
and then initializes etcd. Currently no old data is removed from the object
storage but it's just ignored.
Currently we aren't setting a basepath and it wasn't always correctly handled.
Fix missing basepath handling and improve tests to also use a non empty
basepath.
split data files in multiple files of a max size (default 10Mib)
In this way every data snapshot will change only the datafiles that have some
changes instead of the whole single file.
rename the previous posix storage to posixflat and make it currently not user
selectable (since I'm not sure it's really worth using it).
The new posix storage uses the filesystem without any escaping so it's not a
real flat namespace.
This isn't a real issue since also minio is not a flat namespace and we are so
forced to use it like a hierarchycal filesystem.
* Rename to datamanager since it handles a complete "database" backed by an
objectstorage and etcd
* Don't write every single entry as a single file but group them in a single
file. In future improve this to split the data in multiple files of a max size.
