Allow flush to complete before receiving

When using async socket types we should let the send flush
before receiving the next message.
Added router/dealer perf tests with nextTick changes

Reverted nextTick changes from PR #338
https://github.com/JustinTulloss/zeromq.node/pull/338

Added router/dealer perf tests. Tests fail with buffer isEmpty
in libzmq without the nextTick changes.