Swap Eventbus for Engine to make the trait naming a bit clearer

This will help implementers understand that _they_ must bring the engine to
power the Bus :)

eventbus/src/lib.rs

@@ -39,11 +39,11 @@ pub mod server {
     pub type AsyncOptionMessage = Pin<Box<dyn Future<Output = Option<Message>> + Send + 'static>>;
 
     pub struct Bus {
-        inner: Arc<dyn Eventbus>,
+        inner: Arc<dyn Engine>,
     }
 
     impl Bus {
-        pub fn new(inner: Arc<dyn Eventbus>) -> Self {
+        pub fn new(inner: Arc<dyn Engine>) -> Self {
             Self { inner }
         }
         pub fn pending(
@@ -73,7 +73,7 @@ pub mod server {
     }
 
     /**
-     * The Eventbus trait should be implemented by the servers which need to
+     * The Engine trait should be implemented by the servers which need to
      * implement their own backing stores for an Otto eventbus.
      *
      * Each function is expected to return some sort of future that the caller
@@ -83,7 +83,7 @@ pub mod server {
      * can be used in many cases to help differentiate between consumers of the same
      * topic, such as with Kafka consumer groups
      */
-    pub trait Eventbus {
+    pub trait Engine {
         /**
          * Return the number of messages the caller has not yet consumed
          */

eventbus/src/servers/inmemory.rs

@@ -1,5 +1,5 @@
 /**
- * This is the simplest implementation of an Otto Eventbus, which keeps everything
+ * This is the simplest implementation of an Otto Engine, which keeps everything
  * only in memory
  */
 #[deny(unsafe_code)]
@@ -29,18 +29,18 @@ fn main() -> Result<(), std::io::Error> {
 }
 
 /**
- * The MemoryBus is a simple implementation of the Eventbus trait for a totally
+ * The MemoryBus is a simple implementation of the Engine trait for a totally
  * in-memory eventbus. There is no backing store and all data will be lost between
  * restarts of the application.
  *
- * This is the most simple and primitive implementation of the Eventbus trait
+ * This is the most simple and primitive implementation of the Engine trait
  */
 struct MemoryBus {
     topics: Arc<DashMap<Topic, Vec<Message>>>,
     offsets: Arc<DashMap<(Topic, CallerId), Offset>>,
 }
 
-impl Eventbus for MemoryBus {
+impl Engine for MemoryBus {
     fn pending(
         self: Arc<Self>,
         topic: Topic,