On second thought instead of using log.Panicf(), panic() and do the
logging with recover(). This somewhat centralizes logging in
obfs4proxy, which will be easier to change when I invariably decide to
do logging differently in the future.
This adds preliminary support for data padding by adding another layer
of encapsulation inside each AEAD frame containing a type and length.
For now, data is still sent unpadded, but the infrastructure for
supporting it is mostly there.
Additionally, use log.Panic[f]() instead of panic through out the code
so that some panics are logged.
