Make events with missing parents go through eventcache

[tpapagian]

When we bounce through the event cache its possible we never found the
parent process. In this case we also get a ref cnt imbalance because
the exit event will dec the ref but the exec does not.

To fix this update the Retry logic to also search for the parent
and if its missing populate it plus do refInc. And similarly
on exit check for parent and if its not handled yet similarly
go to the cache and wait, but this time do a refDec.

Now its still possible the dec happens before the inc in this
scenario,

  exec -> cache ------------------> parentFound IncRef
              exit -> decRef

But, in this case its OK that the ref briefly was incorrect because
the GC will wait for at minimum two iterations before removing it
to account for this eventually consistent logic.

Finally, there is the corner case where the exec is delayed for
some extreme amount of time, the GC interval * 3, and then the
exit is handled with its decRef. Now we've go an imbalance. But
we tried really hard here and the system is not responsive so
best we can do is carry on and let the cache reconcile this later.
It could cause some entries to get froze in the cache until the
LRU evicts them. This should be exceedingly rare and we will
add counters for this case which would indicate the GC interval
should be extended.

Signed-off-by: John Fastabend [email protected]
Signed-off-by: Anastasios Papagiannis [email protected]