Refactor Env for AbstractClient:

Env is changed to use the AbstractClient interface,
which generalizes the transport for submitting client
requests to the Env server instance.

The JSONRPCClient implementation is added, supporting
a simple, synchronous interface. Env is changed to
use the JSONRPCClient implementation instead of the
built in JSON-RPC client.

src/ripple/test/AbstractClient.h

@@ -0,0 +1,58 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2016 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef RIPPLE_TEST_ABSTRACTCLIENT_H_INCLUDED
+#define RIPPLE_TEST_ABSTRACTCLIENT_H_INCLUDED
+
+#include <ripple/json/json_value.h>
+
+namespace ripple {
+namespace test {
+
+/* Abstract Ripple Client interface.
+
+   This abstracts the transport layer, allowing
+   commands to be submitted to a rippled server.
+*/
+class AbstractClient
+{
+public:
+    virtual ~AbstractClient() = default;
+
+    /** Submit a command synchronously.
+
+        The arguments to the function and the returned JSON
+        are in a normalized format, the same whether the client
+        is using the JSON-RPC over HTTP/S or WebSocket transport.
+
+        @param cmd The command to execute
+        @param params Json::Value of null or object type
+                      with zero or more key/value pairs.
+        @return The server response in normalized format.
+    */
+    virtual
+    Json::Value
+    invoke(std::string const& cmd,
+        Json::Value const& params = {}) = 0;
+};
+
+} // test
+} // ripple
+
+#endif