Switch from epee http server to boost::beast http server. Min boost 1.70 (#136)

There is roughly a 7.4% increase in performance in the switch to
boost::beast. Additionally, the REST endpoints `/daemon_status`,
`/get_unspent_outs`, and `/submit_raw_tx` do not block in ZMQ calls,
allowing for better response times regardless of `monerod` status.

The REST endpoints `/login and `/get_random_outs` still need updates
to prevent blocking (`/login` is conditional on DB state).

src/rpc/light_wallet.h

@@ -94,7 +94,9 @@ namespace rpc
 
   struct daemon_status_response
   {
-    daemon_status_response() = delete;
+    //! Defaults to current network in unavailable state
+    daemon_status_response();
+
     std::uint64_t outgoing_connections_count;
     std::uint64_t incoming_connections_count;
     std::uint64_t height;