Livestatus: Add 'is_reachable' to hosts/services table.

Refs #6094
DB IDO: Add 'is_reachable' column to {host,service}status tables.

Refs #6094
StatusDataWriter: Add 'is_reachable' {host,service}status attribute.

Refs #6094
DB IDO: Bump schema version to 1.11.3.

Refs #6094
Merge branch 'feature/is-reachable-6094' into next

Fixes #6094