RFR: 8036703: Add trace event with statistics for the metaspace chunk free lists

[Erik Helin]

Hi all,

this patch adds a new trace event - 
vm/gc/metaspace/chunk_free_list_summary for getting statistics for 
metaspace's chunk free lists. The event contains the numbers of chunks 
in the free list and the size of the chunks in the free lists. This 
information is provided for all different kinds of chunks - special, 
small, medium and humongous.

This information will be useful when diagnosing fragmentation issues in 
metaspace.

Webrev:
http://cr.openjdk.java.net/~ehelin/8036703/webrev.00/

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8036703

Testing:
- JFR JTREG tests
- JPRT
- Manually verifying that the numbers look correct by looking at
   recordings from small test cases.

Thanks,
Erik