rpc: Do not reset @ping_started to 0 in ping callback

This is to avoid indefinite recursion of the following kind, that could
lead to a stack overflow:

rpc_clnt_start_ping() -> rpc_clnt_ping() -> rpc_clnt_submit() ->
rpc_clnt_start_ping() -> rpc_clnt_ping() -> rpc_clnt_submit() ...
and so on,

since it is possible that before rpc_clnt_start_ping() is called a
second time by the thread executing this codepath, the response to
previous ping request could ALWAYS come by and cause epoll thread to
reset conn->ping_started to 0.

This patch also fixes the issue of excessive ping traffic, which was
due to the client sending one ping rpc for every fop in the worst case.

Also removed dead code in glusterd.

Change-Id: I7c5e6ae3b1c9d23407c0a12a319bdcb43ba7a359
BUG: 1116243
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/8257
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>