negative caching is obviated by removal cookies

Empty sessions are deleted from stores which means it's not currently
possible to support negative caching as originally thought. However,
such a deletion also issues a removal cookie, which in turn means
deleted sessions will not be requested from behaving clients. In other
words, the need for negative caching should be largely obviated by this
change.

src/lib.rs

@@ -273,11 +273,6 @@
 //! load from the store. By doing so, read-heavy workloads will incur far fewer
 //! roundtrips to the store itself.
 //!
-//! The cache frontend also supports negative caching, meaning that when a
-//! session is not found in the store, the cache will not try to fetch the
-//! missing session from the store in the future. Again, helping to reduce
-//! roundtrips to the store.
-//!
 //! To illustrate, this is how we might use the [`MokaStore`] as a frontend
 //! cache to a [`PostgresStore`] backend.
 //! ```rust,no_run