RFR: 8242596: Improve JarFile.getEntry performance for multi-release jar

[Claes Redestad]

Hi,

please review this patch to improve JarFile.getEntry performance
on multi-release jar files, mainly contributed by Eirik Bjørsnøs.

The main idea is to piggy-back on the scanning of meta-inf entries
we already do during initCEN to scan what versions exists in the
jar file. This then reduces the number of lookups we'll have to do
in typical scenarios.

Webrev: http://cr.openjdk.java.net/~redestad/8242596/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8242596

Testing: tier1-2

Thanks!

/Claes