This deletes the old NAT implementation.

I have verified that UPnP and NAT-PMP work against an older version of
the MiniUPnP daemon running on pfSense. This code is kind of hard to
test automatically.

The unit test hooks were turned on 'in production'.

The discovery RPC protocol does not yet distinguish TCP and UDP ports.
But it can't hurt to do so in our internal model.

Will be back soon. Maybe.

Overview of changes:

- ClientIdentity has been removed, use discover.NodeID
- Server now requires a private key to be set (instead of public key)
- Server performs the encryption handshake before launching Peer
- Dial logic takes peers from discover table
- Encryption handshake code has been cleaned up a bit
- baseProtocol is gone because we don't exchange peers anymore
- Some parts of baseProtocol have moved into Peer instead

crypto.Sha3(append(foo, bar)) --> crypto.Sha3(foo, bar)
crypto.Sha3([]byte{}) --> crypto.Sha3()

- add const length params for handshake messages
- add length check to fail early
- add debug logs to help interop testing (!ABSOLUTELY SHOULD BE DELETED LATER)
- wrap connection read/writes in error check
- add cryptoReady channel in peer to signal when secure session setup is finished
- wait for cryptoReady or timeout in TestPeersHandshake

chop first byte when cryptoid.PubKeyS is set from identity.Pubkey() since this is directly copied in the auth message

- set proper public key serialisation length in pubLen = 64
- reset all sizes and offsets
- rename from DER to S (we are not using DER encoding)
- add remoteInitRandomPubKey as return value to respondToHandshake
- add ImportPublicKey with error return to read both EC golang.elliptic style 65 byte encoding and 64 byte one
- add ExportPublicKey falling back to go-ethereum/crypto.FromECDSAPub() chopping off the first byte
- add Import - Export tests
- all tests pass

- abstract the entire handshake logic in cryptoId.Run() taking session-relevant parameters
- changes in peer to accomodate how the encryption layer would be switched on
- modify arguments of handshake components
- fixed test getting the wrong pubkey but it till crashes on DH in newSession()