TLS support for Light Client

[thanethomson]

At the moment, the Light Client makes use of tendermint::net::Address to keep track of RPC addresses, but this doesn't allow for the full range of RPC functionality introduced in #820 (e.g. TLS and proxy support).

We should update the Light Client to use the new URL type, since it's not really peer addresses the Light Client's interested in, but their RPCs' URLs.

Some preliminary work was done in this commit, but I had to roll it back because it broke the WASM build.

What's the definition of "done" for this issue?

Once we can successfully run the Light Node against nodes with secure endpoints (i.e. HTTPS).