mgmt/osdp: cp: Rework CP state management

With changes to the phy layer and the way we build and dispatch
commands, we can now simplify some parts of CP state machine. This patch
builds on previous patches to reduce some state transitions and fixes
some bugs in those paths.

While at it, also implement an exponential back-off for PD offline after
a timeout. This is useful when there is a transient PD timeout but the
PD itself is not totally dead.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>