refactor(util) move block generator

@whyrusleeping @jbenet

Putting the block generator in a util dir until blocks.

Can't put it in util/testutil because the util/testutil/dag-generator
imports blockservice and blockservice uses the generator.

Tough problem. This'll do for now.

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>
fix(notifications) prevent deadlock when context cancelled early

+ test(notifications)

cc @whyrusleeping @jbenet

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>