DESC
Make sure all references to transaction IDs are properly handled
EDESC

Transaction IDs are unsigned and always greater than 0.

There was a bug that might cause it to be negative because the mmap was casted
to a signed integer and not to an unsigned one.

Besides, start from 0 instead of 1; there's no reason to begin with 1.

Note that the number stored in the lockfile is the maximum number in use, 0
meaning there are no transactions in course. But for no reason a transaction
can have a tid 0.