barkd: guarantee one barkd per datadir

lucarequested to merge
luca/eng-617-barkd-allow-system-to-discover-running-barkd into master
  • check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)

This MR aims at having only one Barkd per datadir. It cannot use only the wallet lock as it needs to hold true before wallet creation and after wallet deletion.

It also modifies behavior on --port 0 (no longer accepts OS assigned ports) but I am in doubt if this will cause test flakiness.

Edited by luca

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