server/server: Remove all connections on shutdown

server/server: Remove all connections on shutdown

This patch fixes a memory leak in the internal server. Steps for
reproduction:

 * valgrind --leak-check=full --show-reachable=yes ./build/src/openocd
 * Establish more than one connection to OpenOCD (e.g. telnet)
 * Shutdown OpenOCD
 * Check for memory leaks in add_connection()

Change-Id: I0ae6fcf2918fd9bdec350446d3e26742d08ff698
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/4053
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>