Ensure that API/JSON-RPC messages in the same session are processed and not stalled

Ensure that API/JSON-RPC messages in the same session are processed and not stalled

This basically drops the "corked" implementation which just stalled the
TLS IO polling after some requests. If you need sort of rate limiting
for these events, use an external TLS proxy which terminates that in front
of Icinga.

fixes #6635