Abundance of usize in xlog_utils and zenith_utils::lsn

[sharnoff]

Currently, postgres_ffi::xlog_utils and zenith_utils::lsn both use usize for many of the integer arguments, even though the functions all converts it to something else later (u32 or u64). I'm sure there's other modules that do this as well, but I'm just highlighting these because I was working with them.

If possible, I think we should be explicit about the size of the integers we're expecting there, because interfacing with these tends to still require casting to a usize and back, because we read and store the data as u32s or u64s. There isn't really a good rust analog for C's "I'll take an integer of any size using #define" -- or at least not one that doesn't heavily add to code verbosity and compile times.

I opened this issue instead of just making the PR because I wanted to gauge people's thoughts/opinions on this.