Because `Map` stores elements in arbitrary order, addition of new
element takes linear time (`Index` iterates over all keys). Thus our
`storageFind` is actually quadratic in time. Optimize this by creating
map from sorted slice.
```
name old time/op new time/op delta
StorageFind-8 157µs ± 2% 112µs ± 1% -28.60% (p=0.000 n=10+10)
name old alloc/op new alloc/op delta
StorageFind-8 69.4kB ± 0% 60.5kB ± 0% -12.90% (p=0.000 n=9+10)
name old allocs/op new allocs/op delta
StorageFind-8 2.21k ± 0% 2.00k ± 0% -9.37% (p=0.000 n=10+7)
```
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
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