Race condition in handle_load_stacks_chain_tip for rpc requests

[pavitthrap]

The function handle_load_stacks_chain_tip (in rpc.rs) is used to load the stacks tip against which an rpc request is executed.
The function will load the latest unconfirmed tip if that is requested. However, it is possible that this tip will be invalidated by the time the it is used in maybe_read_only_clarity_tx.

The consequences of this race would be that the user would receive a 404 response.

If this race condition seems to happen frequently, we can modify maybe_read_only_clarity_tx to reload the unconfirmed tip again if it is found to be invalid.

This race condition was mentioned by @jcnelson in PR #2778

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[jcnelson]

This is not stale; it's a back-burner task.

[jcnelson]

Temporarily assigning to @pavitthrap so this has an owner. Please feel free to reassign.