The once_cell dependency should be optional

[tatsuya6502]

As of moka@v0.12.7, once_cell dependency in Cargo.toml does not have optional = true. However, we removed the global thread pool from moka@v0.12.0 so we do not need once_cell in the most of the time.

We need once_cell only for the followings:

• some unit tests → should be a dev-dependencies.
• the unstable-debug-counters feature → should have optional = true.