Further refactoring of TimeBasedRollingPolicy. Most of the work
related to triggering has moved into an interface called
TimedBasedFileNamingAndTriggeringPolicy. Related to LBCORE-61.

The file attribute in RollingFileAppender is honored once again. This
reverts the changes in revision 2376. Relevant tests in
TimeBasedRollingTest have been resuscitated.