This adds TCP KeepAlive support to client. Close #30, also related to #87. Three arguments are added: keepalive-time, keepalive-interval, keepalive-retries.

Let's say keepalive-time=300s, keepalive-interval=10s, keepalive-retries=3. This feature works like this: when the client detects that no TCP packet has arrived within 300 seconds, it will send a TCP Keep-alive packet to server. If server doesn't respond in time, the client will retry in 10 seconds. After 3 retries, the client will give up this connection. When server received the Keep-alive packet, it will immediately respond with an ACK.

Although the user application may already have its own keep-alive mechanism, they cannot detect broken connections. To actually distinguish broken "TCP" connection, it is still necessary to have keep-alive on fake-tcp layer. For example, if the NAT mapping table on the router is somehow reset, packets are no longer be able to go through. Without keep alive detection, the client will not mark the connection as expired because the UDP client keeps sending packets, so the UDP client may never be able to communicate. With keep alive detection, after the phantun client detected that the connection is not receiving data, it will actively send keep-alive packets. When maximum retries are reached without response, the client knows that the connection is broken, and it will remove the connection from its memory, so a new connection can be established. This feature will not add any extra overhead on active connections, because packets from the user application will prevent the keepalive-time timer from being triggered.