refactor(core): WallClockStore trait replaces the store_mut backdoor

Task 10b reached LogStore::set_wall_clock through a general Node::store_mut.
Kyle: replace it with a narrower accessor. A WallClockStore trait (set_wall_clock
+ peer_time_estimate, implemented only by LogStore) now backs Node::set_wall_clock
and Node::peer_time_estimate, so no &mut Store escapes the facade.

tines-core/src/lib.rs

@@ -25,7 +25,7 @@ pub use node::{Node, ObserveError};
 pub use observation::{
     Dest, NodeId, Observation, ObservationId, Payload, SigAlgo, Signature, Tick, Topic,
 };
-pub use store::{Store, StoreError, TrimPolicy};
+pub use store::{Store, StoreError, TrimPolicy, WallClockStore};
 pub use transport::{Event, FrameBuf, SendError, Transport};
 pub use wire::{frame_sender, Frame, WireError};