Initialize Delta accounts on plugin load

We don't yet configure any of the accounts, but we're set up to manage
several accounts simultaneously in the same plugin instance, using
delta's built-in support for that.

src/messages/handle_proxy.rs

@@ -0,0 +1,44 @@
+use super::{FdSender, SystemMessage};
+use crate::{Handle, ProtocolData};
+use async_std::channel;
+
+pub struct HandleProxy<'a> {
+    pub handle: Handle,
+    pub sender: &'a mut FdSender<SystemMessage>,
+}
+
+impl<'a> HandleProxy<'a> {
+    #[allow(dead_code)]
+    pub async fn exec<F, T>(&mut self, f: F) -> Option<T>
+    where
+        F: FnOnce(&mut crate::PurpleDelta, &mut ProtocolData) -> T,
+        F: Send + 'static,
+        T: Send + 'static,
+    {
+        let (tx, rx) = channel::bounded(1);
+        self.exec_no_return(move |plugin, protocol_data| {
+            if let Err(error) = tx.try_send(f(plugin, protocol_data)) {
+                log::error!("Failed to send result: {:?}", error);
+            }
+        })
+        .await;
+        rx.recv().await.ok().or_else(|| {
+            log::error!("Failed to receive result");
+            None
+        })
+    }
+
+    pub async fn exec_no_return<F>(&mut self, f: F)
+    where
+        F: FnOnce(&mut crate::PurpleDelta, &mut ProtocolData),
+        F: Send + 'static,
+    {
+        self.sender
+            .send(SystemMessage::ExecHandle {
+                handle: self.handle.clone(),
+                function: Box::new(f),
+            })
+            .await;
+    }
+}
+