Loading fresh blobs fails if fresh channel history contains recurring groups

[yegorskii]

This line of code contains bug:
https://github.com/ydb-platform/nbs/blob/main/cloud/blockstore/libs/storage/partition_common/actor_loadfreshblobs.cpp#L111

If channel history contains some group at least twice then we can read some blobs more the once, because both reads will start from barrier event if group generation is greater.

Later during adding blobs to state we will fail here:

https://github.com/ydb-platform/nbs/blob/main/cloud/blockstore/libs/storage/partition/part_state.cpp#L816