Fix filter map race

Fix Eventer race

This commit also documents (but doesn't enforce) that filters
are immutable while they're installed. This required a minor API
change.

This test reports the race condition when run
using "go test -race".

The new event machine will be used for loose coupling and handle the
communications between the services:

1) Block pool finds blocks which "links" with our current canonical
chain
2) Posts the blocks on to the event machine
3) State manager receives blocks & processes them
4) Broadcasts new post block event

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