--enable-lock implemented.

It uses phtread recursive mutexes. Its initialized by
riemann_client_init. I have to drop RIEMANN_CLIENT_INIT, but
riemann_client_init does the exactly same job if --disable-lock is
passed.

A new odd header was included. riemann/_config.h this is the
autoconf generated header. I need it because contains the
RIEMANN_WITH_LOCK macro defined my configure. Another option is create
a riemann/client.h.in, but I'm lazy.

Since I have to drop RIEMANN_CLIENT_INIT this is API version 2 with no
backward compatibility.