MINOR: proto_reverse_connect: emit log for preconnect

MINOR: proto_reverse_connect: emit log for preconnect

Add reporting using send_log() for preconnect operation. This is minimal
to ensure we understand the current status of listener in active reverse
connect.

To limit logging quantity, only important transition are considered.
This requires to implement a minimal state machine as a new field in
receiver structure.

Here are the logs produced :
* Initiating : first time preconnect is enabled on a listener
* Error : last preconnect attempt interrupted on a connection error
* Reaching maxconn : all necessary connections were reversed and are
  operational on a listener