RFR: Refactor runtime allocation to avoid code duplication and upstream diff

[Roman Kennke]

As $SUBJ says.

It reverts most diffs to upstream in memAllocator.hpp|cpp. Only
remaining is to declare class Allocation as protected, otherwise we
can't override mem_allocate()... this needs to be upstreamed.

It also fixes our impl to be saner.

Testing: tier3_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/refactor-rt-allocs/webrev.00/

Roman