Msf::DBManager specs prior to reorganization

MSP-11466

Spec the reachable classes, constants, and methods from Msf::DBManager and its current declaration files (`lib/msf/db_manager.rb` and `lib/msf/db.rb`) so that these very long files can be broken up into Modules that collect together related methods and there is a one-to-one correspondence between Class/Module and file.

# Verification Steps

- [ ] `rake spec`
- [ ] VERIFY no failures