feat(iroh-sync): validate timestamps and move validation up (#1439)

## Description

* Moves validation of entries from the store to `sync.rs`
* Skip validation for locally-inserted entries (not needed, signature
was created by us)
* Reject entries that are more than 10 minutes in the future of our
local device's wall clock time

## Notes & open questions

* The validation uses `SystemTime::now()`. Maybe we could make `Replica`
generic over a `TimeSource`, with a default impl `SystemTimeSource` that
uses `SystemTime`. This would allow tests to plug in a mock time source.
But if its only for tests, it might not be worth it. I don't think
there's many realistic scenarios where you'd want to swap `SystemTime`
for something else - apart from platform support, where `SystemTime` has
a few gaps (e.g. wasm). So maybe let's keep it simple for now and add
the generic once a need outside of tests arise.

* Do we want to make the "max 10 minutes in the future" const
configurable?

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.

Author: Frando