[bitcoin/bitcoin] configurable ZMQ message limit (#13315)

@FeedTheWeb chillax bro, we’re all in this together. i built on your work and, i believe, covered all of the feedback so far on my branch in one commit at https://github.com/mruddy/bitcoin/tree/zmqhwm.

@jmcorgan does my commit cover your feedback? my changes create a configurable hwm option for each notification type. i left the default at 1000 since it’s configurable and people that don’t configure it get the current 1000 messages for all notification types.

if my commit is good, you can use it here, or i can create a separate PR. it doesn’t matter to me.