Make the server decode a pack as it streams.

Make the server decode a pack as it streams.

This, in combination with using recv() instead of read(), makes it so we
never do blocking reads past the end of the pack stream, even when the
client doesn't close the connection.

This is done via a PackStreamVerifier class that reads from a Protocol,
unpacks and counts objects, writes through to a file, and computes the
SHA-1 checksum on the fly. It is necessary because the only way we know
when the pack is supposed to end is by parsing the header and reading
the correct number of objects; otherwise, any further reads from the
client would hang.

Changed the Handler constructors to take a Protocol instead of taking
read and write callbacks separately. Modified some interfaces to utility
functions in pack.py so they can be used by the server-side code.

Change-Id: Id4d11e34658d1f00ad06e45330d0d128b367d8e5