Allow InstrumentedAppender use in logback.xml

eed57bb in Issue #368 got 90% of the way toward allowing an InstrumentedAppender defined in a logback.xml rather than in code. However, the logback.xml appender parser doesn't know how to pass a parameter to the constructor as the InstrumentedAppender currently requires.

This adds a no-arg constructor that uses a shared registry with a default name, or a name defined via system property.
I also converted it from AppenderBase to UnsynchronizedAppenderBase, on the presumption that the registry is itself threadsafe.

This would also allow reversion of 4310891. Let me know if a 3.x-based patch would be better.