failover: add failover priority replica option

[georgiy-belyanin]

Add the failover_priority config option in the] sharding.<replicaset_id>.replicas.<replica_id> section.

Using the option you may specify the order failover will connect to the replicas. The lower the value, the higher the priority of the replica.

[georgiy-belyanin]

Instead of the extra failover configuration section I've implemented an approach with the extra optional shading replica configuration property.

return {
    sharding = {
        ['cbf06940-0790-498b-948d-042b62cf3d29'] = {
            replicas = {
                ['8a274925-a26d-47fc-9e1b-af88ce939412'] = {
                    name = 'storage_1_a',
                    master = true,
                    failover_priority = 1
                },
                ['3de2e3e1-9ebe-4d0d-abb1-26d301b84633'] = {
                    name = 'storage_1_b',
                    failover_priority = 2
                },
                ['001688c3-66f8-4a31-8e19-036c17d489c2'] = {
                    name = 'storage_1_c',
                    failover_priority = 3
                }
            },
        }
    }
}

In my opinion it's better not to manage with the duplicates of the replica sets and replicas UUIDs and to specify the failover priorities near the definition.

Also I haven't found the corresponding issue yet.