This makes the connmgr package an internal package such that it will no
longer be an exported module.  The only place its functionality is
really needed is for the internal implementation of dcrd itself and thus
having an unnecessary exported module significantly increases the
maintenance burden.

It is worth noting that there are a few things, which will be listed
below, that were exported from this module that callers might have
previously relied upon.  These things have been moved to the address
manager module.

The aforementioned methods and types are:

- TorLookupIP method and all associated ErrX constants
- HttpsSeederFilters struct
- DialFunc method
- SeedFilterIPVersion method
- SeedFilterProtocolVersion method
- SeedFilterServices method
- SeedAddrs method

Overview of the major changes:

- Move the following files from connmgr -> internal/connmgr:
  - README.md
  - connmanager.go
  - connmanager_test.go
  - doc.go
  - dynamicbanscore.go
  - dynamicbanscore_test.go
  - error.go
  - error_test.go
  - log.go
- Remove connmgr/go.mod and connmgr/go.sum
- Make the README.md and docs.go files match the new reality
- Update all import paths in the repository accordingly
- Remove the dependency from the root go.mod
- Run go mod tidy on all modules