Avoid unneeded database compaction at startup, and improve backups.

Avoid unneeded database compaction at startup, and improve backups.

Until now, Open vSwitch "start" has always converted the database to the
current database schema.  This compacts the database, which as a side
effect throws away useful information about the transactions that were
executed to bring the database into its current state.  This can make
debugging database-related problems more difficult.

This commit changes the "start" command to only convert the database if
the database schema has changed.  It also adds the database checksum to
the backup file name, to avoid overwriting backups in the case where the
checksum changed but the developer neglected to update the version number.

I tested an earlier version of the xenserver changes but not any version
of the Debian changes.