Add block cache trait (#1192)

zcash_client_backend/CHANGELOG.md

@@ -39,6 +39,7 @@ and this library adheres to Rust's notion of
   - `WalletSummary::next_orchard_subtree_index`
   - `chain::ChainState`
   - `chain::ScanSummary::{spent_orchard_note_count, received_orchard_note_count}`
+  - `chain::BlockCache` trait
   - `impl Debug for chain::CommitmentTreeRoot`
 - `zcash_client_backend::fees`:
   - `orchard`
@@ -54,6 +55,7 @@ and this library adheres to Rust's notion of
   - `Nullifiers::{orchard, extend_orchard, retain_orchard}`
   - `TaggedOrchardBatch`
   - `TaggedOrchardBatchRunner`
+  - `testing` module
 - `zcash_client_backend::wallet`:
   - `Note::Orchard`
   - `WalletOrchardSpend`

zcash_client_backend/Cargo.toml

@@ -84,10 +84,14 @@ shardtree.workspace = true
 
 # - Test dependencies
 proptest = { workspace = true, optional = true }
+jubjub = { workspace = true, optional = true }
 
 # - ZIP 321
 nom = "7"
 
+# - Asychronous
+async-trait = "0.1.78"
+
 # Dependencies used internally:
 # (Breaking upgrades to these are usually backwards-compatible, but check MSRVs.)
 # - Documentation
@@ -116,6 +120,7 @@ shardtree = { workspace = true, features = ["test-dependencies"] }
 zcash_proofs.workspace = true
 zcash_address = { workspace = true, features = ["test-dependencies"] }
 zcash_keys = { workspace = true, features = ["test-dependencies"] }
+tokio = { version = "1.21.0", features = ["rt-multi-thread"] }
 
 time = ">=0.3.22, <0.3.24" # time 0.3.24 has MSRV 1.67
 
@@ -139,6 +144,7 @@ orchard = ["dep:orchard", "zcash_keys/orchard"]
 ## Exposes APIs that are useful for testing, such as `proptest` strategies.
 test-dependencies = [
     "dep:proptest",
+    "dep:jubjub",
     "orchard?/test-dependencies",
     "zcash_keys/test-dependencies",
     "zcash_primitives/test-dependencies",