Lazily create encoder for IncomingMessage

Approximately 2/3 of the initialization time of `new IncomingMessage` is taken
by the `createEncoder` call within IncomingMessage's constructor.

The vast majority of IncomingMessage usage is to decode the incoming message
first to determine if it is relevant--i.e. to determine what document it
belongs to.

This change improves performance by avoiding the `createEncoder` call until
we know it is actually necessary.

packages/server/src/IncomingMessage.ts

@@ -24,17 +24,23 @@ export class IncomingMessage {
   /**
    * Access to the reply.
    */
-  encoder: Encoder
+  private encoderInternal?: Encoder
 
   constructor(input: any) {
     if (!(input instanceof Uint8Array)) {
       input = new Uint8Array(input)
     }
 
-    this.encoder = createEncoder()
     this.decoder = createDecoder(input)
   }
 
+  get encoder() {
+    if (!this.encoderInternal) {
+      this.encoderInternal = createEncoder()
+    }
+    return this.encoderInternal
+  }
+
   readVarUint8Array() {
     return readVarUint8Array(this.decoder)
   }