write handlers, instead the read_packet-handler has a reference
directly to the connection object.

Session key generation halfway written.

Rev: src/client_keyexchange.c:1.3
Rev: src/client_keyexchange.h:1.2
Rev: src/connection.h:1.11
Rev: src/keyexchange.c:1.8
Rev: src/keyexchange.h:1.9
Rev: src/publickey_crypto.c:1.7
Rev: src/publickey_crypto.h:1.6
Rev: src/read_packet.c:1.8
Rev: src/read_packet.h:1.5