Increment conventional call numbers on restart_call()

[taclane]

Conventional calls are assigned a call number when they are first created, and continue to reuse that same call_num for every subsequent call.

While this reuse has no negative technical effects, it is not ideal from the standpoint of logging or reporting to have non-unique numbers as a call identifier.

This patch simply increments the call_num on each Call_conventional::restart_call().

[robotastic]

Brilliant!