This takes care of a few minor nits on the recently merged subscribe
code. In particular:
- Avoid extra unnecessary allocation on notifications slice
- Avoid defer overhead on notification mutex in sendNotifications
- Make test function comment start with the name of the function per
Effective Go guidelines
- Use constant for number of subscribers in test
- Don't exceed column 80 in test print
The BlockChain struct emits notifications for various events, but
it is only possible to register one listener. This changes the
interface and implementations to allow multiple listeners.