Disable explicit GC by default

We don't rely upon GC to cleanup mappedbytebuffers, we unmap them
explicitly on close in lucene. But the JDK has crazy loops with
explicit GCs in exceptional cases to try to force unmapping.

In general we don't want any of our code or library code calling
this method: so its banned in forbidden-apis as well.
Added minimal setup guide for BW Compat tests