[Blockstore] should add blobs to index in parallel while writing them

[SvartMetal]

Currently, WriteBlocks request consists of two consecutive stages:

• Writing a blob to the Blob Storage
• Adding the blob to the partition tablet index

This approach needs improvement. The idea is simple:

1. Persisting 'unconfirmed blob' information in parallel during writing the blobs to Blob Storage
2. Deferring or returning a retriable error for 'unconfirmed ranges' at this time
3. Confirming the blobs by adding them to the index when it's known that all the blobs have been successfully written to Blob Storage
4. Finally, cleaning up the 'unconfirmed blobs', previously persisted in step 1

The only subtlety is what to do with partially-written blobs. Suppose the tablet is restarted in the middle of writing data. In this case, some blobs may be partially written and cannot be recovered. Therefore, we need to confirm/recover these blobs at tablet startup. This recovery process is done using the TEvBlobStorage::TEvGet request with the 'restore' flag for each blob.

[SvartMetal]

The majority of the work has been completed, but there are still some tasks to be done. Must have:

• GetChangedBlocks, DescribeBlocks requests should take this feature into account ✅
• Limit 'unconfirmed' blob count ✅
• Each ConfirmBlobs operation should be visible in profile log ✅
• Support enabling/disabling via 'features config' ✅
• Add sensors about 'unconfirmed' data/blobs ✅

Nice to have:

• Persist 'unconfirmed' blob checksums
• Improve ReadBlocks request (with 'unconfirmed ranges') as much as possible (caching might be used here)
• Batch AddBlobs request during AddConfirmedBlobs operation (might be premature optimization, so it requires a performance test)

[SvartMetal]

Performance improvement on 128 KB workload, iodepth=40:

[qkrorlqr]

Implemented and partially deployed. Works fine.