Add new epoch event handler to GC that finds all expired tombstones and
marks them and underlying objects to be removed. Shard uses callbacks
provided by the storage engine to mark underlying objects.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
In previous implementation each operation on local storage
locked engine mutex. This was done under the assumption that
the weights of the shards change as a result of write operations.
With the transition to static weights of shards, it is no longer
necessary to lock the global mutex during the execution of operations.
However, since the set of engine shards is dynamic, there is still a
need to control multiple access to this set. The same mutex is used
for synchronization.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
