Make Scilla compatible with Zilliqa 2

[JamesHinshelwood]

• We add another binary which exposes the scilla-server JSON-RPC interface over HTTP.
• The state IPC calls now make requests over HTTP, rather than via a unix domain socket. Note that this is a breaking change, meaning this version of Scilla will NOT work with Zilliqa 1.
• All bytes in IPC calls are encoded into base64. Previously, we would include invalid UTF-8 strings in JSON-RPC requests, which resulted in invalid JSON.

We probably don't want to merge this into master, since it breaks Zilliqa 1