RFR (S): 8215472: Cleanups in implementation classes of jdk.zipfs and tests

[Langer, Christoph]

Hi,

when working with jdk.zipfs, I found some opportunity for cleanups. I'd like to contribute this independently from my other thread regarding Posix permissions in zip files (http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-November/056426.html). So, please help to review the cleanup.

Webrev: http://cr.openjdk.java.net/~clanger/webrevs/8215472.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8215472

Thanks
Christoph