Share the transport between locations using the same upstream

This is an important commit that fixes connection overload problem in case of hundreds of locations.
Currently each location uses it's own transport to the backend. This commit changes this behavior to reuse the same transport
from the upstream. This change also moves transport-level settings like timeouts to the upstream.
Address code review comments
Merge pull request #122 from mailgun/alexander/transport

Share the transport between locations using the same upstream