Make once_cell dependency optional

It will be used only when the followings:
- development build (e.g. `cargo test`).
- `unstable-debug-counters` feature is enabled.

Cargo.toml

@@ -38,13 +38,12 @@ atomic64 = []
 # This unstable feature adds `GlobalDebugCounters::current` function, which returns
 # counters of internal object construction and destruction. It will have some
 # performance impacts and is intended for debugging.
-unstable-debug-counters = ["future"]
+unstable-debug-counters = ["future", "once_cell"]
 
 [dependencies]
 crossbeam-channel = "0.5.5"
 crossbeam-epoch = "0.9.9"
 crossbeam-utils = "0.8"
-once_cell = "1.7"
 parking_lot = "0.12"
 smallvec = "1.8"
 tagptr = "0.2"
@@ -68,13 +67,17 @@ futures-util = { version = "0.3.17", optional = true }
 # Optional dependencies (logging)
 log = { version = "0.4", optional = true }
 
+# Optional dependencies (unstable-debug-counters)
+once_cell = { version = "1.7", optional = true }
+
 [dev-dependencies]
 actix-rt = "2.8"
 ahash = "0.8.3"
 anyhow = "1.0.19"
 async-std = { version = "1.12", features = ["attributes"] }
 env_logger = "0.10.0"
 getrandom = "0.2"
+once_cell = "1.7"
 paste = "1.0.9"
 reqwest = { version = "0.11.11", default-features = false, features = ["rustls-tls"] }
 tokio = { version = "1.19", features = ["fs", "io-util", "macros", "rt-multi-thread", "sync", "time" ] }