MINOR: connection: make the last arg of subscribe() a struct wait_event*

The subscriber used to be passed as a "void *param" that was systematically
cast to a struct wait_event*. By now it appears clear that the subscribe()
call at every layer is well defined and always takes a pointer to an event
subscriber of type wait_event, so let's enforce this in the functions'
prototypes, remove the intermediary variables used to cast it and clean up
the comments to clarify what all these functions do in their context.
10 files changed