[JENKINS-11416] Added comment describing the motivation of the change. The real fix is in: ccdbd834

Adding the ant-launcher jar to the maven plugin.  On my environment without it gmaven is unable to locate the org.apache.tools.ant.launch.AntMain class

[JENKINS-11458] Channel.current() is only available from the thread that's executing the remote call, and that's why we saw this original error, when Maven uses a thread pool to call into this method from another worker thread.

Fix minor typo in javadocs.

[JENKINS-11458] when building with Maven 3, Channel.current() inside FilterImpl sometimes returns null. Therefore pass channel into constructor, thus effectively restoring pre-1.435 behaviour.

java.lang.NoSuchMethodError: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;Ljava/lang/Throwable;)V
    at org.apache.commons.logging.impl.SLF4JLocationAwareLog.trace(SLF4JLocationAwareLog.java:107)
    at org.apache.commons.beanutils.BeanUtilsBean.convert(BeanUtilsBean.java:1073)
    at org.apache.commons.beanutils.BeanUtilsBean.copyProperty(BeanUtilsBean.java:437)
    at org.apache.commons.beanutils.BeanUtils.copyProperty(BeanUtils.java:160)
    at org.apache.commons.beanutils.ConvertingWrapDynaBean.set(ConvertingWrapDynaBean.java:67)

[JENKINS-10989] Repeated ids, expandTextArea() and multiple "Invoke Ant" build steps

workaround.

- suppressed/fixed several compiler warnings