Blockchain's subscriptions, unsubscriptions and notifications are
handled by a single notificationDispatcher routine. Thus, on attempt
to send the subsequent event to Blockchain's subscribers, dispatcher
can't handle subscriptions\unsubscriptions. Make subscription and
unsubscription to be a non-blocking operation for blockchain on the
server side, otherwise it may cause the dispatcher locks.
To achieve this, use a separate lock for those code that make calls
to blockchain's subscription API and for subscription counters on
the server side.
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