Upgrade notes:
- PostgreSQL replaces MySQL for monitor-api, monitor-scorer and the selector; the agent works with both server versions
- log_scores.rtt is NULL, not 0, for timeouts and implausible round trip times

Server:
- Score a response with a zero or negative round trip time as a bad host (error "implausible rtt", step -4, no offset)
- Leave timeouts out of the average rtt used to rank monitors
- Fix the selector taking a second demotion slot for blocked monitors when trimming the testing pool
- Fix monitorsettings going back to defaults when a refresh fails to parse
- Fix monitor-scorer returning nil when its setup fails
- Fix a data race on the MQTT client list in the metrics endpoint

Client:
- Report a sample with a zero or negative round trip time as an error
- Fix a failed traceroute stalling the rest of the batch
- Fix a nil pointer dereference on an empty config response
- Fix ad-hoc NTP checks changing the sample count of regular checks

Build:
- Update dependencies (go.ntppool.org/common v0.10.3) and Alpine to 3.24.1