RFR (M) 8183923: Get rid of FreeBlockDictionary and dithering

[Mikael Gerdin]

Hi all,

Please review this cleanup inspired by looking at Roman's CMS cleanup :)

FreeBlockDictionary is an old abstraction for multiple CMS freelist 
datastructures which never appear to have been implemented, getting rid 
of it also simplifies some code in Metaspace so it's not all CMS stuff.

Testing: jprt
Bug: https://bugs.openjdk.java.net/browse/JDK-8183923
Webrev: http://cr.openjdk.java.net/~mgerdin/8183923/webrev.0/index.html

Thanks
/Mikael