RFR: 8220496: Race in java_lang_String::length() when deduplicating

[Claes Redestad]

Hi,

a few asserts in java_lang_String can trigger when deduplicating
Strings. The fix is to ensure we assert using a deeper equals on the
contents of the value.  Moving the method already implemented to do this
in StringDedupTable to java_lang_String seems to be in order, along with
a few cleanups.

Webrev: http://cr.openjdk.java.net/~redestad/8220496/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8220496

Testing: tier1-3

Thanks!

/Claes