added reqpub pingpong endpoint

[dwsutherland]

sibling of cylc/cylc-uisever

Also as per cylc/cylc-uiserver#857, added ZeroMQ socket options to increase connect/reconnect timeouts.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry not needed (it's on the UIS end)
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.