Correct issue with pushing address messages.

The code to send an address messages in batches was previously clearing
all addresses from the existing message after queueing it to be sent.
Since the message is a pointer, this means it was removing the addresses
from the same message which might not have already been sent yet (from
another goroutine) which led to a race.

This commit modifies the code to create a new address message for each
batch as intended.

Fixes #58.