Allow application filters on smart HTTP operations

Allow application filters on smart HTTP operations

Permit applications embedding GitServlet to wrap the
info/refs?service=$name and /$name operations with a
servlet Filter.

To help applications inspect state of the operation,
expose the UploadPack or ReceivePack object into a
request attribute.  This can be useful for logging,
or to implement throttling of requests like Gerrit
Code Review uses to prevent server overload.

Change-Id: Ib8773c14e2b7a650769bd578aad745e6651210cb
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>