style: factor out handle_packet
support fragmentation of handshake packets, addresses #29
require handshake fragment to be empty at the end of a handshake
remove dec_resp from handshake_handler return type, because that can only ever happen in response to
a change cipher spec message (which is handled by the ccs_handler)
pull out separate_handshakes
Merge pull request #89 from mirleft/internal-fragmentation

Internal fragmentation