cache: investigate using xsync.RBMutex

[`cache.shard`](https://github.com/cockroachdb/pebble/blob/master/internal/cache/clockpro.go#L79) currently provides mutual exclusion using a `sync.RWMutex`. We should investigate whether [`xsync.RBMutex`](https://github.com/puzpuzpuz/xsync?tab=readme-ov-file#rbmutex) provides better scalability under concurrent access. `xsync.RBMutex` is optimized for read-lock acquisition and for cache-like workloads where readers are much more frequent than writers.

Jira issue: PEBBLE-536

Start in internal/cache/clockpro.go at cache.shard, where sync.RWMutex currently provides mutual exclusion. Read the xsync.RBMutex documentation, then compare the two mutexes under concurrent cache-like access; done means recording whether RBMutex provides better scalability and documenting the result.

Written by the indexing model from the issue text.