Introduce re-openable log record handler

This patch adds a new log handler class based on the standard library's
BaseRotatingHandler. This new class allows the log file to be re-opened,
e.g. upon receiving a SIGHUP signal. The latter will be implemented in
forthcoming patches. The patch does not change the behaviour regarding
writing to /dev/console.

Quite a bit of code had to be changed to unittest the log handlers.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>