refactor: bind heart methods .beat and .alive

This allows the functions to maintain access to the Heart instance (or
`this`) even when they are passed to other functions. We do this because
we pass both `isActive` and `beat` to `heartbeatTimer`.

Author: jsjoeio

src/node/heart.ts

@@ -9,7 +9,10 @@ export class Heart {
   private heartbeatInterval = 60000
   public lastHeartbeat = 0
 
-  public constructor(private readonly heartbeatPath: string, private readonly isActive: () => Promise<boolean>) {}
+  public constructor(private readonly heartbeatPath: string, private readonly isActive: () => Promise<boolean>) {
+    this.beat = this.beat.bind(this)
+    this.alive = this.alive.bind(this)
+  }
 
   public alive(): boolean {
     const now = Date.now()