When a session is continuously used, for example in a multi-tier setup,
it can take a very long time before an HTTP/1 session runs out of tasks
and unwinds back to the point where the worker might update its stats.

This phenomenon is amplified by thread_stats_rate, so it might take even
longer to get statistics published. This is famously observed with load
testing campaigns where a certain set of VSCs may not increase at all
until the very end of a load run.

This change makes it possible for HTTP/1 sessions to at least publish
their statistics into the pool every thread_stats_rate requests.

To avoid a potential performance cost, this behavior (technically
complying with the documentation) is guarded by a feature flag. The
flag is generic so this behavior may be extended to other busy loops,
present (if any) or future.