The dec_buf seems to be properly managed between read calls,
and we have no logic to decrypt before attempting socket I/O.
Thus - until now - such data would not be decrypted in case of
connections such as HTTP keep-alive, as the recv call would
always get executed first, block until rw_timeout, and then get
retried by retry_transfer_wrapper.

Thus - if data is received - decrypt all of it right away. This way
it is available for the following requests in case they can be
satisfied with it.