Update `object#config_hash` after all relations queries

[yhabteab]

The IDO is writing the checksums of all Host or/and Service attributes, including group memberships, to the database when the object is first created or, one of its attributes e.g. enable_flapping_detection changes. However, when either the Icinga 2 or the MySQL service is stopped/reloaded unexpectedly right after the IDO has written the checksums to the database but before the remaining queries of the group memberships are executed, the group members will never be written to the database until someone modifies one of the object attributes.

This PR defers the config_hash column update until all object relationships have been updated/inserted.

Master:

• Upsert all object config fields including config_hash column, and ...
• Update/Insert all relations such as hostgroup_members, servicegroup_members etc.

This PR:

• Upsert all object config fields including config_hash, but config_hash is set to NULL
• Update/Insert all relations such as hostgroup_members, servicegroup_members etc.
• Update object config_hash column to the current in memory value.

closes #8813 ref/IP/53530

[Al2Klimov]

I.e., before: SendConfigUpdateHeavy() updates the object itself, calls OnConfigUpdateHeavy(). Have fun with race conditions.

After: SendConfigUpdateHeavy() NULLs config hash, calls OnConfigUpdateHeavy(), updates the object itself.

[yhabteab]

Just added some additional comments!