start_watching starts a regular async job (via Clock.every) that increments a counter, and a C thread to make sure that the counter is incremented in a timely manner.
val dump_core : ?how_to_dump:How_to_dump.t-> unit -> unit
dump_core () dumps a core file using /usr/bin/gcore if it exists, or by calling abort() if not (or with ~how_to_dump:Call_abort). With gcore, the dump is done in a child process, and the core file is written to /tmp/core.$N.$PID, where $PID is the process id and $N is a counter that is incremented on each call to dump_core.