Allow wallet2.h to run in WebAssembly

- Add abstract_http_client.h which http_client.h extends.
- Replace simple_http_client with abstract_http_client in wallet2,
message_store, message_transporter, and node_rpc_proxy.
- Import and export wallet data in wallet2.
- Use #if defined __EMSCRIPTEN__ directives to skip incompatible code.

src/wallet/message_store.cpp

@@ -48,7 +48,7 @@
 namespace mms
 {
 
-message_store::message_store()
+message_store::message_store(std::unique_ptr<epee::net_utils::http::abstract_http_client> http_client) : m_transporter(std::move(http_client))
 {
   m_active = false;
   m_auto_send = false;