types: unify all epoch timestamps to i64

All epoch timestamp fields (timestamp, last_replayed, created_at on
nodes; timestamp on relations) are now i64. Previously a mix of f64
and i64 which caused type seams and required unnecessary casts.

- Kill now_epoch() -> f64 and now_epoch_i64(), replace with single
  now_epoch() -> i64
- All formatting functions take i64
- new_node() sets created_at automatically
- journal-ts-migrate handles all nodes, with valid_range check to
  detect garbage from f64->i64 bit reinterpretation
- capnp schema: Float64 -> Int64 for all timestamp fields

src/migrate.rs

@@ -216,9 +216,10 @@ pub fn migrate() -> Result<(), String> {
             uses: old_entry.uses,
             wrongs: old_entry.wrongs,
             state_tag,
-            last_replayed: 0.0,
+            last_replayed: 0,
             spaced_repetition_interval: 1,
             position: 0,
+            created_at: 0,
             community_id: None,
             clustering_coefficient: None,
             degree: None,
@@ -255,9 +256,10 @@ pub fn migrate() -> Result<(), String> {
             uses: 0,
             wrongs: 0,
             state_tag: unit.state.clone().unwrap_or_default(),
-            last_replayed: 0.0,
+            last_replayed: 0,
             spaced_repetition_interval: 1,
             position: 0,
+            created_at: 0,
             community_id: None,
             clustering_coefficient: None,
             degree: None,