Even under heavy load the server now properly purge dead workers.

When we started to receive and process messages in less than 1 millisecond
we would fail to purge dead workers. The reason for this is that we asked
the polling monitor to return in milliseconds the duration from the last
time we polled. And when this was sub 1 millisecond we would get 0 back.

So now we use a independent time tracking to see that 250ms have passed
since the last time we polled.

Change-Id: Iea91d919086674b20555940998070500d8553d10
Remove the duration* methods from PollingMonitor as they are unused.

With the server not using these methods to track when to purge dead workers,
we can now safely remove the methods all together.

Change-Id: I130edb35dbeb9aee06b0eead61624a1b4e7a58ee
Merge branch 'fast_polling_ignores_dead_workers'

Conflicts:
	remus/server/Server.cxx

Change-Id: Ib5fd9319d113bd7bc92c182321c399100db13dc4