client: libuv: fix close handling during redirect

During client redirect we "reset" the wsi to the redirect address,
involving closing the current fd that was told to redirect (it will
usually be a completely different server or port).

With libuv and its two-stage close that's not trivial.  This solves
the problem we will "reset" (overwrite) where the handle lives in the
wsi with new a new connection / handle by having it copied out into
an allocated watcher struct, which is freed in the uv close callback.

To confirm it the minimal ws client example gets some new options, the
original problem was replicated with this

$ lws-minimal-ws-client-echo -s invalid.url.com -p 80

https://github.com/warmcat/libwebsockets/issues/1390