RFR: Rename enter/exit_critical to pin_object

[Roman Kennke]

Hi,

Christine suggested to rename enter_critical() and exit_critical() in
CollectedHeap (and related) to pin_object() and unpin_object(). I agree
that this is a more useful name and this change implements that:

http://cr.openjdk.java.net/~rkennke/rename-critical/webrev.01/

Ok?

Roman