[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.
Merge remote-tracking branch 'upstream/1.9-maint' into 2.0-maint

Conflicts:
	enterprise/backup/src/main/java/org/neo4j/backup/BackupService.java
Merge pull request #3211 from davidegrohmann/merge-1.9-into-2.0

Merge 1.9 into 2.0