span: btreeFrontier iterator protection should use counter

The `btreeFrontier` currently uses an atomic bool to prevent writers from modifying the frontier while an iterator on it is open. However, if there are multiple readers (which is theoretically possible in a single-threaded application via pull iterators), the bool could be reset back to false after the first reader completes at which point, writers would be allowed.

https://github.com/cockroachdb/cockroach/blob/3fb64cbd7e030b004b4bba20fd2a6d1da3b2a4f7/pkg/util/span/frontier.go#L538-L557

We could replace the bool with a counter if we want to allow multiple readers or we could simply disallow multiple readers.

Related to #151217

Jira issue: CRDB-53123

Read pkg/util/span/frontier.go around lines 538-557 and review the related issue #151217. Determine whether multiple pull-iterator readers should be supported or disallowed, then ensure writers remain blocked for the full lifetime of any open iterator. The issue does not name a test to run or add.

Written by the indexing model from the issue text.