blockchain: always relock chainLock for subscription callbacks

[kcalvinalvin]

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.

[coveralls]

Pull Request Test Coverage Report for Build 8060178922

Details

• 0 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 56.732%

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Totals
Change from base Build 8057525961:	0.007%
Covered Lines:	29202
Relevant Lines:	51474

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💛 - Coveralls