workerpool: Additional check in BaseWorker.ShouldTerminate
workerpool: Additional check in BaseWorker.ShouldTerminate

Document that it should only be called from within RunTask and
add an assertion for this. This means we can no longer use a
method on the pool and hence remove WorkerPool.ShouldWorkerTerminate.

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

mbakke/ganeti,andir/ganeti,grnet/snf-ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,leshchevds/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,grnet/snf-ganeti,apyrgio/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,onponomarev/ganeti,dimara/ganeti,onponomarev/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,ganeti/ganeti,andir/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,leshchevds/ganeti,dimara/ganeti,andir/ganeti