Msf::DBManager reorganization

MSP-11124

Merge together `lib/msf/core/db.rb` and `lib/msf/core/db_manager.rb` into `lib/msf/core/db_manager.rb` so that the class name matches the file name.  Break up `lib/msf/core/db_manager.rb` into `Module`s of related methods that are included back into `Msf::DBManager`.  This drops the the 7771 lines making up the Msf::DBManager class into 68 files with no more than 600 lines in anyone file.  This breakup points out the repetiveness of some of our importers also, which can be used as guidance for later refactoring to use metaprogramming to get rid of this duplication.

**NOTE: These changes should only involve moving methods between different files/Modules.  If you spot a change to a method body, that is an error and should be removed.**

# Pre-verification Steps
- [x] Merge #3971

# Verification Steps

## specs
- [x] `rake spec`
- [x] VERIFY no failures

## cucumber
- [x] `rake cucumber`
- [x] VERIFY no failures