Performance-test Dispatcher.sinksFor() function

[mjstrasser]

The function Dispatcher.sinksFor() calculates the current set of sinks every time a log event is dispatched. This might be very inefficient with large numbers of log events.

Perhaps introduce a simple cache (map) of sinks, keyed by logger name. Considerations:

• What if there is a very large number of logger names? Could the map grow very large? (YAGNI?)
• How often might the configuration change? Level configurations can be changed by calling the config DSL in a running application. Perhaps introduce simple cache invalidation? (YAGNI?)

[mjstrasser]

b9b5211 created a simple map cache.