sql/sessioninit: use syncutil.RWMutex for the cache

[knz]

Describe the problem

During a postmortem we've found that the syncutil.Mutex in sessioninit.Cache may be a source of contention when there is a high rate of session establishments.

We can reduce this by using syncutil.RWMutex instead.

Jira issue: CRDB-15599

[knz]

cc @rafiss

I had a brief look into it. I found it a bit complicated to achieve: the path that clears the cache if the versions don't match must be done "atomically" with the path that uses the cache. The code needs to be reorganized a bit to achieve this.