1.6.fix warnings

The code compiles now with -w Ae and -warn-error A.
the checksum is *verified* whenever an entry is read from file (in tlogcommon.ml). 
If can check it again, but that means you don't trust the checksums on tcp level. 
Something can be said for it (as that one is a 16 bit checksum over a different range of data), but I would not go that far.