drbd: Concurrent write detection fix

Commit 9b1e63e changed the concurrent write detection algorithm to only insert
peer requests into write_requests tree after determining that there is no
conflict.  With this change, new conflicting local requests could be added
while the algorithm runs, but this case was not handled correctly.  Instead of
making the algorithm deal with this case, switch back to adding peer requests
to the write_requests tree immediately: this improves fairness.

When a peer request is discarded, remove that request from the write_requests

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>