DRY up duplicate transport code

[c24t]

#256 copied the transports package from trace/ into common/. We now have two nearly-identical copies of the code. See common/transports and trace/exporters/transports.

Before merging these we need to check that the new default options for stats transports will work for tracing.

[ocervell]

Any plans to merge the trace/exporters/transport and common/transports soon ? How do we ensure this will not break backwards compatibility ?

[c24t]

@ocervell no specific plans, and we have some work to do to make sure the tests are sufficient to prove that we don't break backwards compatibility. Luckily the code is almost identical in both places.