This brought unnecessary complexity into the decision procedures while the
performance aspects weren't worth it. It just saved one ea_list traversal
when many others are also done.

The "dump *" commands are totally garbled by the time annotations.

The change 371eb490 introduced early free
of old_config. Unfortunately, it did not properly check whether it is not
still in use (blocked by obstacle during reconfiguration). Fix that.

It also means that we still could have a short peak when three configs
are in use (when a new reconfig is requeste while the previous one is
still active).

Instead of propagating interface updates as they are loaded from kernel,
they are enqueued and all the notifications are called from a
protocol-specific event. This change allows to break the locking loop
between protocols and interfaces.

Anyway, this change is based on v2 branch to keep the changes between v2
and v3 smaller.

This makes for safer and faster pruning and notifying as protocol now on
its shutdown prunes only its neighbors and nothing else.

The interface list must be flushed when device protocol is stopped. This
was done in a hardcoded specific hook inside generic protocol routines.
The cleanup hook was originally used for table reference counting late
cleanup, yet it can be also simply used for prettier interface list flush.

Instead of calling custom hooks from object locks, we use standard event
sending mechanism to inform protocols about object lock changes. This is
a backport from version 3 where these events are passed across threads.

This implementation of object locks doesn't use mutexes to lock the
whole data structure. In version 3, this data structure may get accessed
from multiple threads and must be protected by mutex.

Instead of calling custom hooks from object locks, we use standard event
sending mechanism to inform protocols about object lock changes. As
event sending is lockless, the unlocking protocol simply enqueues the
appropriate event to the given loop when the locking is done.

This reverts commit 7144c9ca.

The onlink attribute implementation collides with the nexthop attribute
behavior in v3; keeping it aside until finding out how to reimplement it
correctly.