annotation block size should be configurable

[stuarteberg]

This is a known issue, I'm just writing it down so we don't forget.

The annotation datatype doesn't allow the user to specify the storage block size upon creation. Instead, it assumes the block size of whichever labelmap instance it is sync'd to, or defaults to a block size of 32 if no sync exists.

The block size should be configurable upon instance creation. And if the implementation requires it, forbid users from posting a sync to any labelmap with a mismatched block size.