blockchain: always relock chainLock for subscription callbacks #2128
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For various b.sendNotifcation() callbacks, if a runtime panic happens, we don't get any useful debugging information since the error that happens first is the "unlock of unlocked mutex" error.
This is because we temporarily unlock the chainLock for callbacks and then relock them. However, since the relocking code is executed after the completion of the callback, if an error happens during that callback, we never relock the chainLock.
Switching to an anonymous function and having the unlock code as a defer will ensure that the lock always relocks.