don't deadlock on shutdown

Specifically, when we tear down IO threads, we should take care to
dispose connection.

closes #1775

crates/ra_lsp_server/src/main.rs

@@ -66,7 +66,7 @@ fn run_server() -> Result<()> {
         workspace_roots,
         initialize_params.capabilities,
         server_config,
-        &connection,
+        connection,
     )?;
 
     log::info!("shutting down IO...");

crates/ra_lsp_server/src/main_loop.rs

@@ -52,7 +52,7 @@ pub fn main_loop(
     ws_roots: Vec<PathBuf>,
     client_caps: ClientCapabilities,
     config: ServerConfig,
-    connection: &Connection,
+    connection: Connection,
 ) -> Result<()> {
     log::info!("server_config: {:#?}", config);
 
@@ -174,7 +174,7 @@ pub fn main_loop(
                 &pool,
                 &task_sender,
                 &libdata_sender,
-                connection,
+                &connection,
                 &mut world_state,
                 &mut loop_state,
                 event,

crates/ra_lsp_server/tests/heavy_tests/support.rs

@@ -118,7 +118,7 @@ impl Server {
                         experimental: None,
                     },
                     ServerConfig { with_sysroot, ..ServerConfig::default() },
-                    &connection,
+                    connection,
                 )
                 .unwrap()
             })