Fix transfer statistics extraction

Make the regular expressions used for extracting transfer statistics from a named log file slightly more precise in order to prevent them from matching the wrong lines as this may trigger false positives for the ixfr and xfer system tests.

As an example, the regular expression responsible for extracting the number of bytes transmitted throughout an entire zone transfer could also match a line containing the following string:

transfer of '<zone-name>/IN': sending TCP message of <integer> bytes

However, such a line is not one summarizing a zone transfer.

Also remove superfluous arguments for the get_named_xfer_stats() function in the ixfr and xfer system tests.