Make heartbeat interval configurable.

[jevolk]

Removes hardcoded global; resolves FIXME.

• Link to issue, e.g. Resolves #NNN
• Documentation added (if applicable)
• Tests added
• Branch rebased on top of current main (git pull --rebase origin main)
• Changes squashed to a single commit (described here)
• Build is green in Travis CI
• You have certified that the contribution is your original work and that you license the work to the project under the Apache 2 license

Resolves

nats-server/server/events.go

Lines 83 to 84 in 039f63e

	// FIXME(dlc) - make configurable.
	var eventsHBInterval = 30 * time.Second

Tangentially related to #4227 #4234

[wallyqs]

@jevolk could you try to change the target to be the dev branch? There would only be a minor conflict in const.go