a4edb168b635b6f5c36324e44961cd42cf9bbbaa ZMQ: add options to configure outbound message high water mark, aka SNDHWM (mruddy)
Pull request description:
ZMQ: add options to configure outbound message high water mark, aka SNDHWM This is my attempt at https://github.com/bitcoin/bitcoin/pull/13315
Backport of Core PR14060 https://github.com/bitcoin/bitcoin/pull/14060/commits/a4edb168b635b6f5c36324e44961cd42cf9bbbaa