Add integration tests for exp module

[aisk]

The exp clients were only covered by unit tests against a scripted in-process server; add integration tests running against a real memcached.

• tests/test_exp.rs: set/get/delete, store modes, CAS flow (including unless_cas), arithmetic with vivify, item metadata, binary keys (base64 path), run_batch, the full lease flow and me debug, plus an async roundtrip behind the tokio feature
• Add a tokio dev-dependency (rt + macros) for #[tokio::test]
• CI now runs the whole suite once with --all-features, covering the tokio tests in the same step