(PDB-100) Convert terminus to internally use a URL connecting to PDB

There purely a refactor of terminus' HTTP code to use URLs rather than
the existing server + port combination internally. The user still
specifies server+port and we convert this internally to URLs. This is
step one in getting connection failover for the terminus.

This is my first significant change to the terminus code and would appreciate some feedback on the new API, style etc.
We talked about this in hipchat a bit, but if I understand right we don't currently know how often commands might be lagging for 30+ seconds because the current behavior is just to continue waiting.  I've been running the benchmark tool under pretty heavy load for the last 30 minutes and haven't seen any timeouts yet but that isn't conclusive.  Like we discussed it seems like the cost of raising the limit to 2 minutes is pretty low under the assumption that this is a rare event to begin with.