Shared concurrent heartbeat() promise

[t-d-d]

All concurrent heartbeat() calls will return the same promise object. Proposed solution to #1025

Added a test that reproduces #1025 but more tests needed.

[tulios]

@t-d-d thanks a lot for the PR. I left some comments. They are basically suggestions; the only one that I would like to see changed is making the utility a little less dense, so it's easier to grasp.

[tulios]

Do we need defaultFunc? It's currently not used anywhere. I would also use curly braces and make this a little bit less dense, wdyt?

[t-d-d]

I think you are right - it is trying to be too clever. I had both options because it's slightly easier to use the 'invocation time' method from a class (well, less code changes are needed to adopt it.)

I have changed to only provide option at initialisation time. Let me know what you think.

[t-d-d]

Actually this shouldn't be merged until we conclude the issue of what to do with failed heartbeats that @Nevon raises in #1025 .
My thinking is that there's no point in queuing up heartbeat calls that will get issued one-by-one when they fail. I presume that in most cases, if one fails they are all going to fail.