Allow explicitly overridding global ZBlk format for individual ZBigFile and ZBigArray

Hello Kirill,

the following patches allow setting the ZBlk format of individual ZBigFile and ZBigArray to a different value than the system-wide global value. This is useful in cases where we have an instance on which we do have some data which is better suited in a format different from most of the other data on this instance.

Regarding the implementation, it's a bit unfortunate that this patch needs to add "zblk_fmt" to ZBigFile's state and therefore making it less concise, but I wouldn't know how else we could make the user declared format persistent. I attempted to make this change backwards compatible e.g. to support ZBigFile states with two items or with three items in order to use an old DB with a newer version. I also added tests to ensure the changes are effective. Both make test.py and make test.go succeed on my instance.

Best, Levin