chore(deps): bump memcache from 0.20.0 to 0.21.0

[dependabot]

Bumps memcache from 0.20.0 to 0.21.0.

Release notes

Sourced from memcache's releases.

v0.21.0

What's Changed

This release focuses on the experimental memcache::exp module. The low-level MetaClient from 0.20 is replaced by a high-level client whose verbs hide CAS tokens and leases from caller code. The stable memcache::Client is unchanged.

exp is still experimental and may break between minor versions. Pin the minor version if you depend on it:

[dependencies]
memcache = "0.21"

New high-level client: exp::Memcache and exp::AsyncMemcache

• Blocking scenario client Memcache with verbs named for what you are doing: get, set, delete, counters, update (read, compare and swap, retry loop handled internally) and fetch (compute a missing value once, with dogpile protection via leases)
• AsyncMemcache on tokio, behind the tokio feature, with the same verb set
• Scenario layer built on plan / finish functions in the exp core so both clients share the same logic
• Raw meta commands remain reachable through cache.meta()
• Pre-connect builder: timeout, connect_timeout, max_idle, max_idle_age, max_connections, degrade, on_error, hash_function, router
• Router trait for custom placement, with rendezvous hashing as the default
• Failure policy (degrade) and an on_error observability hook
• Keys accept anything AsRef<[u8]>; client_flags override on writes

Types

• New Ttl, Freshness, Encode / Decode traits and a dedicated exp::Error type
• Protocol layer now works on Ttl and raw byte values
• New serde_json feature: serialize values to JSON via serde

Batches

• run_batch reports per-operation results instead of a single outcome
• Typed run_many batch entry point on both clients
• Async batches exchange server groups concurrently, bounded by one deadline
• Partial batch responses are order-checked and opaque tokens are verified

Connection handling

• Idle connections are probed before reuse and dropped past max_idle_age
• Per-server connection cap (max_connections) with token give-back on failure
• Redial on stale connections, but not after a timeout
• IO timeout applied as one deadline per exchange in both clients
• Ambiguity attribution: errors distinguish whether a write was fully sent
• Bounded VA length when parsing values
• Routing uses the configured address rather than the resolved one

API renames

• MetaClient replaced by Memcache / AsyncMemcache
• get_touch renamed to get_and_touch

... (truncated)

Commits

• 3be7531 🔖 Bump version to 0.21.0
• 75d7871 📝 Add a Simplified Chinese version of the exp guide
• 4849aef 📝 Split the exp guide into docs/exp.md and rework the README feature list
• 1ee0b1b ✅ Add exp scenario layer integration tests and drop redundant unit tests
• 62398f1 ♻️ Rename exp get_touch to get_and_touch
• b4305f6 📝 Restructure the exp README section to match the Python client docs
• 6af48ef 🐛 Fix exp review findings: bounded VA length, order-checked partial batches, ...
• 1d0783b 📝 Document the exp scenario layer in the module docs and README
• 57fc446 ✨ Harden the exp engine: idle age, connection cap, redial, ambiguity attribut...
• b688d6e ♻️ Route exp keys through a Router trait with rendezvous hashing by default
• Additional commits viewable in compare view