Adds optional TCP_KEEPALIVE to WebSocketServer

TCP_KEEPALIVE is now enabled by default. Settings for
KEEPCNT, KEEPINTVL and KEEPIDLE can be supplied when
creating WebSocketServer and KEEPALIVE can also be
disabled if required.

Also adds new unit test for testing.

Co-authored-by: natsume.takashi@lab.ntt.co.jp
Merge pull request #93 from dosaboy/topic/set-keepalive-options

Adds optional TCP_KEEPALIVE to WebSocketServer