Ensure correct working when node contains multiple replicas of single vdisk

[ikopylov]

It is perfectly valid to place vdisk replicas on a single node, but on different physical disks. Currently, Virtual::get_operation (https://github.com/qoollo/bob/blob/master/bob-common/src/mapper.rs#L185) returns only the first physical disk containing vdisk replica. Thus quorum (https://github.com/qoollo/bob/blob/master/bob/src/cluster/quorum.rs#L35) and backend (https://github.com/qoollo/bob/blob/master/bob-backend/src/core.rs#L243) writes data only to that one replica and always skips the other ones