Fixes #50774

When the server boots up, 2 threads hit the same `UnboundTemplate`
instance before it has set up `@templates`. Both threads get past the
`unless template = @templates[locals]` check because
`@templates[locals]` isn't set yet. However, with `@write_lock`, one
thread waits while the other one proceeds, setting `@templates` to a
frozen hash. The second thread then gets the write lock and tries to
modify `@templates` but it has been frozen.