Migrate master tables to flatbuffers

[oberstet]

Currently, even though we do have a properly defined schema

• https://github.com/crossbario/cfxdb/blob/master/cfxdb/mrealm.fbs

we only have CBR-based zLMDB tables, which follow those schemata by convention, but use CBOR as value-type

• https://github.com/crossbario/cfxdb/blob/master/cfxdb/mrealmschema.py
• https://github.com/crossbario/cfxdb/blob/master/cfxdb/mrealm/management_realm.py

We should migrate those tables to proper Flatbuffers value-typed tables. This should be (almost) without any impact on the existing code in cfx master! at least it is a good test of this promise;) as in, "how it should work as designed" ..