Remove Executor trait from collab and use an enum instead

This will let us save off the executor and avoid using generics.

crates/collab/src/lib.rs

@@ -2,6 +2,7 @@ pub mod api;
 pub mod auth;
 pub mod db;
 pub mod env;
+mod executor;
 #[cfg(test)]
 mod integration_tests;
 pub mod rpc;