optional write aptwebserver log to client specific files

The test test-handle-redirect-as-used-mirror-change serves multiple
clients at the same time, so the order of the output is undefined and
once in a while the two clients will intermix their lines causing the
grep we perform on it later to fail making our tests fail.

Solved by introducing client-specific logfiles which we all grep and
sort the result to have the results more stable.

Git-Dch: Ignore

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