decommissioned ReconfigureOnChangeFilter and replaced it with  ReconfigureOnChangeTask, added ScheduledExecutorService to Context

Changed the default enable status of packaging info in stacktraces
to be disabled to avoid the expensive lookup unless requested.

Added an XML attribute "packageTrace" to <configuration> in order to
allow enabling or disabling the packaging info in stacktraces. When
the attribute is omitted, the default setting is assumed (disabled).

To enable package detail in logback.xml:

  <configuration packageTrace="true">
    ...
  </configuration>

LOGBACK-730
LOGBACK-966

LOGBACK-833 - allow previously defined properties (in local scope) to be seen by SiftingJoranConfigurator
 * implemented part two of Ceki's suggestion
 * add getCopyOfPropertyMap() to the PropertyContainer and thus InterpretationContext
 * use local properties that are ineffect at time of creation of SiftingAppender to seed the properties map used when creting the child appenders and properties
 * removed some generics warnings
 * in AppenderFactoryBase use List.subList() rather than "copy whole array then delete and re-shuffle elements"