feat: oci tar format, bit-perfect disk storage for config and manifest, concurrent image pulls (#88)

* oci: retain bit-perfect copies of manifest and config on disk

* feat: oci tar format support

* feat: concurrent image pulls

Cargo.lock

@@ -1452,6 +1452,7 @@ dependencies = [
  "log",
  "prost",
  "redb",
+ "scopeguard",
  "signal-hook",
  "tokio",
  "tokio-stream",