Refactor host/port uri generation

Tools need to generate a URI from various host/port combinations:

noformat
--host foo # single host
--host foo --port 27017 # single host and port
--host foo,bar # multiple host
--host foo,bar --port 27107 # multiple host plus port
--host foo:27017,bar:27107 # multiple host/port pair (but maybe invalid without repl set name?)

1. For all the above, host can be prefixed with a replica set name

--host setname/foo,bar --port 27107

1. It's also possible to specify a port by itself

--port 33333

Logic to handle this is scattered, including:

• options/options.go (ParseArgs)
• db/db.go (configureClient)
• helpers in util/mongo.go

These should all be centralized and rationalized to convert any host/port combination into the expected URI and there should be unit testing for the conversions.