Extract classes for transport within a JGit process

Extract classes for transport within a JGit process

TransportLocal knows how to spin up a thread to allow two repositories
in the same process to communicate using the wire protocol. However,
it is still tied to local on-disk filesystems, and needs to be able to
fork processes if not using the default git-{upload,receive}-pack
implementation.

Extract out the connection classes so they can be used by other
transport implementations.

Change-Id: I5db59086740735508c2e70a597c2d1a89014b072