Use Guards to Ensure That Only One Request Creates or Deletes an App

This commit ensures that subsequent DELETE requests of the same app name wait
for the results of the first DELETE request. This makes the DELETE idempotent.
Additionally, it ensures that POST and DELETE requests use the same guard based
on the app name.

api-tests/Cargo.toml

@@ -12,6 +12,6 @@ lazy_static = "1.4"
 reqwest = { version = "0.10", features = ["json"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
-testcontainers = { git = "https://github.com/testcontainers/testcontainers-rs.git", branch = "master" }
+testcontainers = { version = "0.9" }
 tokio = { version = "0.2", features = ["macros"] }
 uuid = { version = "0.8", features = ["serde", "v4"] }