Improve overlay connection

[gianfra-t]

Addresses #580 and #568.

Changes

• Config can now take an array of node values. It will pick one randomly.
• Add a timeout and a health check on the receiver side of overlay_conn

[ebma]

Code looks good overall 👍 I think this might work, let's try it next week.

[gianfra-t]

@ebma the tests seem to have passed except of course the ones that always fail, as it was already failing on the 1.6 PR. I think this could be related to the original issue, hard to tell. Shall we release this first and tackle that later?

[ebma]

Sure, let's go.