RFR 8152271: MemberNameTable doesn't purge stale entries

[Coleen Phillimore]

Summary: Intern MemberNames in table instead of allocating new entries

For degenerate case, we were leaking native code in the member name 
table.  Going with the suggested workaround, it was only a few more 
lines of code to intern MemberName and return the MemberName that was 
already in the table.

This has been performance tested to show no regression and works really 
well for the degenerate test case, even though the real percentage of 
reused MemberName seems quite small.

Tested with all runtime nightly tests, tests in jdk/test/java/lang/invoke.

open webrev at http://cr.openjdk.java.net/~coleenp/8152271.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8152271

Thanks,
Coleen