BUG/MEDIUM: mux-h1: Fix a deadlock when a 408 error is pending for a client

BUG/MEDIUM: mux-h1: Fix a deadlock when a 408 error is pending for a client

When a frontend H1 connection timed out waiting for the next request, a 408
error message is returned to the client. It is performed into the H1C task
process function, h1_timeout_task(), and under the idle connection takeover
lock. If the 408 error message cannot be sent immediately, we wait for a
next retry. In this case, the lock must be released.

This bug was introduced by the commit c4bfa59f1d ("MAJOR: mux-h1: Create the
client stream as later as possible") and is specific to the 2.4-DEV. No
backport needed.