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