In close cooperation with Stefan, I refactored and rewrote the MIDAS event
buffering code (bm_send_event, bm_flush_cache, bm_receive_event and bm_push_event).

The main goal of this update is to make sure the event buffering code does not
have any infinite loops: in the past, we have seen mlogger and some frontends
loop forever consuming 100% CPU in the event buffering code. This should now be
completely fixed.

As additional bonuses, the refactored code is easier to read, has less code
duplication and should be more robust. A few potential logical problems have
been corrected and one case of reproducible infinite looping has been fixed.

The new code has passed the low-level consumer-producer tests, but has not yet
been used in anger in any real experiment. One hopes any new bugs introduced
would cause outright failures and core dumps (rather than silent data corruption).

All are welcome to try the new code. If it explodes, please send me the error
messages, stack traces and core dumps.

K.O.