Re: Rewrite of JAR plugin ready for merge

Hello Sergey

I pushed a new commit on 
https://github.com/apache/maven-jar-plugin/pull/508 which execute `jar 
--validate` only if requested. Can you check if it resolves the 
performance issue if you have an opportunity?

For all: I added a new configuration parameter in the Maven JAR Plugin, 
named "validate" with the following documentation:

    Whether to validate the JAR files after their creation. If `true`,
    the `jar` tool is executed a second time with
    the `--validate` option for each created JAR file. This verification
    may slow down the build if these files are large. If `false` (the
    default), there is no second jar execution. If `auto`, the Maven JAR
    Plugin will decide itself whether to execute `jar --validate` based
    on heuristic rules.

I'm not sure about the "auto" option and would be okay to remove it if 
peoples prefer. Its rational was to avoid an unnecessary validation when 
the JAR was already implicitly validated by `jar --release`. But on a 
second though, `jar --validate` may be also unnecessary on a JAR which 
is not multi-release.

     Martin