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
I wonder if we can actually pass that `InputStreamDataInput` directly to `TranslogStreamVersionX` ctors instead of opening and closing them... we can simply seek backwards no?