RFR: 8210889: Some service thread cleanups can be starved

[Kim Barrett]

Please review this small change to the ServiceThread.  When checking
for work to be done, rather than immediately doing the first kind of
work found, instead determine all pending work and then do all known
pending work.  This avoids the possibility of frequently available
work early in the sequence of checks from leading to starvation of
work associated with later checks.

CR:
https://bugs.openjdk.java.net/browse/JDK-8210889

Webrev:
http://cr.openjdk.java.net/~kbarrett/8210889/open.00/

Testing:
mach5 tier1-3