Wait for outgoing transfer statistics to be logged

Wait for outgoing transfer statistics to be logged

Since the message confirming outgoing transfer completion is logged
asynchronously, it may happen that transfer statistics may not yet be
logged by the time the dig command triggering a given transfer returns.
This causes false positives for the "ixfr" and "xfer" system tests.
Prevent this from happening by checking outgoing transfer statistics up
to 10 times, in 1-second intervals.