opStrategy and ordering for ArithmeticPostAgg

Fixes #510
make prioritized executor service listenable
allow forcing all tasks to be prioritized
remove initial delay from polling

remove initial delay from polling in coordinator, this will help in the
cases when the pollDuration is set to higher than coordinator
startDelay.
review comment
Merge pull request #537 from metamx/remove-initial-delay-polling

remove initial delay from polling
fix jetty threads getting stuck

Fix Jetty threads getting stuck on exceptions
make MetricsEmittingExecutorService listenable
use Guava's ForwardingExecutorService instead of DelegatingExecutorService
fix backwards compat issue
Merge branch 'master' of github.com:metamx/druid
fix broken CombiningSequence.toYielder behavior
tests must not assume findOvershadowed ordering
always use ByteCountingLRUMap.remove() to remove
Merge pull request #538 from metamx/fix-jetty-exception-handling

fix jetty threads getting stuck
union query fix and a broken test
clean up event receiver firehose factory
Merge branch 'master' of github.com:metamx/druid
add sanity unit tests
add header