When watching many (5000) actors at the same time the
following problems were found:
* first send of a sys msg is sent without any flow control
=> limit the number of outstanding sys msg by using
the buffer to send them later (ordinary resend)
* when msg cannot be written sys msg is dropped (relying on resend),
but that cause message re-ordering and negative acknowledgment,
which is very costly
=> buffer the sys msg on write failure
=> minor optimization of AckedReceiveBuffer
I also made the resend-limit configurable.
(cherry picked from commit ecfc271e9a9d7efcf76945632d89c78740291cc6)
dec53381b6