There is a need to support working w/o shard if it has problems with
blobovnicza tree.
Make `BlobStor.Init` to return new `ErrInitBlobovniczas` error. Remove
shard from storage engine's shard set if it returned this error from
`Init` call. So if some of the shards (but not all) return this error,
the node will be able to continue working without them.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Add blobovnicza instance to BlobStor structure. Create blobovnicza tree in
BlobStor constructor. Implement Open/Init/Close methods.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
