RFR(M): 7163191: G1: introduce a "heap spanning table" abstraction

[John Cuthbertson]

Hi Everyone,

Here's another change from Tony that I've been maintaining since he left 
Oracle. Can I have a volunteer look over the changes? They look good to 
me. The webrev can be found at: 
http://cr.openjdk.java.net/~johnc/7163191/webrev.0/

Summary:
Currently in G1 there are several tables where a each element is used to 
hold some information about a fixed size subdivision of the heap. For 
example the HeapRegion table and the fast CSet test tables. This change 
encapsulates the base properties of these tables in a generic heap 
spanning table and uses the abstraction for the heap region sequence 
table. A future change will extend this to the fast cset test arrays.

Testing consisted of the GC test suite with a low marking threshold and 
heap verification. Testing did catch an assertion failure that I have 
included a small fix for. As with the additional marking validation 
changes, if no-one objects, I will push this change with Tony as the 
author and myself as a reviewer.

JohnC