Make `BlockExecution` / `ResumeExecution` to not release per-shard worker
pools. Make `StorageEngine.Close` to block these methods and any
data-related operations. It is still releases the pools.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
There were no unit tests of storage engine. This commit
adds first test to reproduce missing link ID in split info
at `engine.Head(raw)` request.
Engine tests uses some constructors from metabase tests,
so it is better to locate such functions in common
package at local_object_storage.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
