1) handle 64 bit file offsets in the token code. I wonder how large
bit files worked up till now?

2) send a null token when we have passed over a large lump of data
without finding a token match. This reduces the number of IOs
considerably as it removes the need for seeks/reads on the checksum
calculation and literal send code. This is not enabled yet for the
compressed case as the deflate token code can't handle it yet.