Extract protocol v2 fetch request parsing to its own class

Extract protocol v2 fetch request parsing to its own class

The parsing code for protocol v2 fetch doesn't have any dependency on
the rest of UploadPack.

Move it to its own class. This makes testing easier (no need to
instantiate the full UploadPack), simplifies the code in UploadPack and
increases modularity.

At the moment, the parser needs to know about the reference database to
validate incoming references. This dependency could be easily removed
moving the validation later in the flow, after the parsing, where other
validations are already happening. Postponing that to keep this patch
about moving unmodified code around.

Change-Id: I7ad29a6b99caa7c12c06f5a7f30ab6a5f6e44dc7
Signed-off-by: Ivan Frade <ifrade@google.com>