shutdown of the server

[cendhu]

There are three ways to perform a shutdown:

1. Just kill the main program abruptly which would kill all goroutines as well.
2. Finish all the pending transactions, blocks before shutting down the peer but no new connections would be allowed.
3. Stop all goroutines immediately and then the server.

For (1), just a CTR + C is good enough. For (2) and (3), we need to add code to support the graceful shutdown while (1) is more work than (2).