[Idea] Detect and warn about clock skew when performing silo monitoring

While discussing the fixes in #7519, @benjaminpetit mentioned that we could do more to detect and warn about clock skew, eg by including timestamps in our inter-silo ping messages.

We could decide to warn if clock skew is greater than, eg, 30 seconds or perhaps one minute.

Start by reading the fixes discussed in #7519 and locating the inter-silo ping messages used by silo monitoring. Clarify the clock-skew threshold and warning behavior before changing anything. Done means skew can be detected from ping timestamps and a warning is emitted when the agreed threshold is exceeded.

Written by the indexing model from the issue text.