(TK-27) Allow user to manage clients explicitly

This PR will allow the user to explicitly create instances of a sync or async client, and then use those clients for multiple http requests. All the previous APIs remain intact; if the user uses the old http request functions, things will behave as they did before (i.e., it will create a new client and close it once the response comes in).
I vote for keeping the protocol fn sigs as close as humanly possible to the 'regular' function sigs.  Personally I don't have any problem with the `common/` namespacing here.