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>
