S = Server
C = Client

For binary data, we use the chunked encoding like what's defined in
RFC2616.  The format is: "$size_of_chunk\r\n$chunk_data".  We assume
that all data, filenames, signatures and delta are all binary.

C: SIG\r\n$filename\r\n$signature
"I have $filename with $signature"
S: (OK|MISSING|NEWER|DELETE)
if OK:
   File already exists in the same version on the server
if MISSING:
   The server does not have a record of the file (so please upload it)
   C: PUT $filename\r\n$delta
   Delta is relative to an empty file.
if NEWER:
   The server has a newer version of the file
   C: DELTA $filename $signature
   "Please give me the delta of $filename, from $signature"
   S: $filename\r\n$delta
if DELETE:
   The server has a record of the file being deleted, and the client
   should do likewise.

XXX: Conflicts.  Client-side cache of signatures?  Needs "generation"
on server too.