Make concurrent GC breakpoints more specific

During the review of JDK-8289739 there has been the comment that the "after" and "before" prefixes of the breakpoint name could both describe points in time as well as periods.

The suggestion has been to replace the breakpoint names with "at-XXX-start" and "at-XXX-end".

Further the placement of these breakpoint calls may be reconsidered in G1 as currently they are placed in unexpected places.