Factor a base class out of ReceivePack

We are working on a publish/subscribe based git protocol, and we want to
reuse several parts of the ReceivePack-like code for reading commands
and processing a pack. In this new implementation, the connection
management will be very different, in particular, there may be multiple
packs received on a single open connection. So, hoist out as much as we
can from ReceivePack, mostly just leaving behind the single-connection
version in that class.

Change-Id: I5567aad6ae77951f73f59c1f91996d934ea88334