Add translog checksums

Switches TranslogStreams to check a header in the file to determine the
translog format, delegating to the version-specific stream.

Version 1 of the translog format writes a `0xffff_ffff_0000_0001` header
at the beginning of the file and appends a checksum for each translog
operation written.

Also refactors much of the translog operations, such as merging
.hasNext() and .next() in FsChannelSnapshot

Relates to #6554
So even if the 1.4 node has a 1.3 translog, it can calculate and send the checksums for the operation to another 1.4 node, they just won't be in the actual file (which I think is pretty sweet!). This doesn't deal with the header check so it's okay.