Log error when a cached response is not found
Allow for response cache inspection
Send cached response on request retransmission (WIP)
Refactor wrapping data with BOSH <body/>
Extract pick_handler from send_to_handler
Allow send_to_handler to take already wrapped data
Send cached response on request retransmission (done)
Adjust debugging to current situation
Allow to inspect the backtrace on conversion failure

Catching the exception will most probably cause a badmatch and show a
backtrace. Otherwise the information where a badarg happens is hidden,
because the reported location is always inside `ejabberd_binary` module.
Refactor caching control (WIP)