Remove support for Write deadlines since it wasn't working.

Write timeouts are obnoxious to handle as the frame encoder state
already is updated to cover the entire payload for the Write() call
that timed out.  In theory it is possible to buffer the pending data,
but that causes Write() to voilate the semantics of the interface.
Preliminary support padding, log on panic.

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.