RFR: 8210753: Make ThreadLocalAllocBuffer::resize() public

[Per Liden]

Make ThreadLocalAllocBuffer::resize() public to allow a GC to resize 
TLABs in parallel. With resize() public I propose that we remove 
ThreadLocalAllocBuffer::resize_all_tlabs() and let 
CollectedHeap::resize_all_tlabs() iterate over the threads, just like 
CollectedHeap::ensure_parsability() does.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210753
Webrev: http://cr.openjdk.java.net/~pliden/8210753/webrev.0

/Per