This moves some of the packet-specific stuff to the packet class itself

This means that meterpreter instances that don't support will continue
to work.

Both extended and verbose session logging will show which of the
sessions has the encryption enabled as it's not yet supported on all
sessions.

Due to the way python3 does things differently, the escaped char string
for the session guid didn't work in the way it was expected. Instead,
we're going to do what we did with the payload UUID and hex encode, and
use binascii to handling the translation for us.

This changes the encryption flags on the meterpreter session so that
it's 32 bits (and hence changes the packet header). This also supports
the idea that sessions may use encryption that isn't AES256, so the
flags field will ultimately indicate that. A type flag has been added so
that MSF knows the type that should be done on the wire.

At some point soon we'll add something that makes sure that the packet
encryption type always matches the encryption type expected in MSF, this
will hopefully avoid the risk of having packets injected into the stream
by external entities.