Avoid discarding pending items of different types

The BotRunner currently discards pending items if a newer one comes in that cannot run concurrently with the pending one. This is based on the idea that they will do the same work. This assumption is no longer correct after the PullRequestBot avoids running its three different types of WorkItems concurrently. The discard check should take the actual class of the WorkItem into consideration as well before discarding obsoleted ones.