[1.9] Fix backup service incorrectly initializing new log with -1 as previous txId.

The backup service receives a transaction (the last one) and should initialize
the log header with its txId minus one.
Merge pull request #3206 from martinfurmanski/1.9-backup-incorrect-log-header-previous-txid

[1.9] Fix backup service incorrectly initializing new log with -1 txId.