WL#3297: Make SocketClient have internal buffer for gets()

See http://lists.mysql.com/commits/6549

This patch uses the Operating System's buffer and a non-blocking socket to
implement read a line at a time for a socket. Since this is only used through
SocketClient, we could have our own buffer inside SocketClient and use it. This
would reduce even further the number of system calls made my ndb_mgmd and also
keep it compatibily with systems that don't support (or have buggy) non blocking
sockets.

Good beginner task.