plugin pubsub cache: avoid race condition by retrying node request: When retrieving items, there may be a race condition if a node is not found in cache, then created before `setPubsubNode` is called. This patch handle this case by retrying in case of UNIQUE constraint violation.

[DEFAULT]
xmpp_domain = server1.test
email_server = libervia-backend.test
email_port = 8025
email_admins_list = admin@server1.test

[component file-sharing]
http_upload_public_facing_url = http://localhost:7777
http_upload_connection_type = http
http_upload_port = 7777