[NBS] Split read requests in the mirror partition

[komarevtsev-d]

Imagine mirror-3 disk with this config:

┌───────────────────────────────────────────────────────┐
│ uuid-1 (Fresh)   │ uuid-3           │ uuid-5 (Fresh)  │
│──────────────────┼──────────────────┼─────────────────│
│ uuid-2           │ uuid-4 (Fresh)   │ uuid-6 (Fresh)  │
└───────────────────────────────────────────────────────┘

When the user attempts to read a range across two devices, the mirror partition will reply with E_INVALID_STATE because this range is "fresh" for all replicas.
As a solution, we should detect these situations in the mirror partition, split such requests into two parts and glue the result back as one response.