ACCUMULO-2343 Add AsyncSocketAppender

AsyncSocketAppender is a Log4J AsyncAppender with its own internal SocketAppender.
Configuration for either appender can be set on the AsyncSocketAppender itself. An
AsyncSocketAppender can be configured using a Log4J properties file, while an ordinary
AsyncAppender cannot.
Merge branch '1.6.1-SNAPSHOT'
ACCUMULO-2383 Add log4j properties support

Accumulo now looks for either XML or properties files for the Log4J configuration. It will
also heed the log4j.configuration isystem property if it is set. Its MonitorLog4jWatcher
can now load either properties or XML.
Merge branch '1.6.1-SNAPSHOT'