p2p: integrate p2p/discover

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

p2p/testlog_test.go

@@ -15,7 +15,7 @@ func testlog(t *testing.T) testLogger {
 	return l
 }
 
-func (testLogger) GetLogLevel() logger.LogLevel { return logger.DebugLevel }
+func (testLogger) GetLogLevel() logger.LogLevel { return logger.DebugDetailLevel }
 func (testLogger) SetLogLevel(logger.LogLevel)  {}
 
 func (l testLogger) LogPrint(level logger.LogLevel, msg string) {