Added possibility to add a user body handler inside a file body handler to dynamically edit the content of the file in both directions

- rename belle_tls_verify_policy into belle_tls_crypto_config (but backward compatibility ok)
- add macro to get compile warnings on deprecated functions usage
- add possibility to give the ssl_configuration structure to the crypto library
   (works with mbedtls 2.2.1 or above only)

fix tester https_get_long_body to point to a more reliable address (I.E https://smtp.linphone.org/linphone.html)

* implement retry of EWOULDBLOCK returned send()
* rework channel's listener for clarity
* fix various memory leaks.

progress in http implementation