Simplify hashed bins config

Coming from #99 (comment 1347977844), which is responding to #99 (comment 1335467920)...

As it stands, in Implement support for hashed bins (#99 - closed), we ask administrators to configure the number of bins they want. However, there's no clear guidance to determine how many bins would be appropriate (see: Properly describe hashed bin delegation in the specification)

PEP 458 discusses Metadata Scalability, which seems relevant to this problem.

Thanks to @lukpueh for the pointer towards Hash_table#Load_factor.