drbd: Delay/reject other state changes while establishing a connection

Changes to the role and disk state should be delayed or rejected
while we establish a connection.

This is necessary, since the peer will base its resync decision
on the UUIDs and the state we sent in the drbd_connect() function.

The most prominent example for this race is becoming primary after
sending state and UUIDs and before the state changes to C_WF_CONNECTION.

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