Stores checksum of last transaction in neostore record

This means that there is no longer a need for having a "last transaction"
in a transaction log for the sole purpose of extracting a checksum from
its start entry. This simplifies usages around backup and HA.

This commit also changes the HA protocol, the handshake specifically,
where previously the handshake contained a checksum and a masterId for
that transaction. That wasn't necessary to have since that masterId value
was and is already part of the checksum.