Handle one message at time on callback interface

On each invocation of 'Connection#parse', a list and an iterator are
allocated. Remove these allocations by handling one incoming message
per invocation on the callback interface.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #2 from jvirtanen/features/connection-callback-message

Handle one message at time on callback interface