fix: transform streams could possibly hang (#29)

This works around a bug where you cannot call done twice in a single event loop
https://github.com/spion/promise-streams/blob/master/index.js#L70-L73
We've been able to observe this bug in production but not reproduce it in tests
A more performance solution would be a queue of some sort because we don't always have to wait until next tick

lib/transform.ts

@@ -3,10 +3,15 @@ import { IBluestream } from './interfaces'
 import { defer, maybeResume } from './utils'
 
 async function transformHandler (data, encoding, done) {
+  // This works around a bug where you cannot call done twice in a single event loop
+  // https://github.com/spion/promise-streams/blob/master/index.js#L70-L73
+  // We've been able to observe this bug in production but not reproduce it in tests
+  // A more performance solution would be a queue of some sort because we don't always have to wait until next tick
+  const finish = () => setImmediate(done)
   const processed = this.asyncTransform(data, encoding)
-    .then(async transformedData => {
+    .then(transformedData => {
       if (transformedData !== undefined) {
-        await this.push(transformedData)
+        this.push(transformedData)
       }
       this.queue.delete(processed)
     }, e => {
@@ -15,9 +20,9 @@ async function transformHandler (data, encoding, done) {
   this.queue.add(processed)
 
   if (this.queue.size < this.concurrent) {
-    return done()
+    return finish()
   }
-  Promise.race(this.queue).then(() => done(), e => this.emitError(e))
+  Promise.race(this.queue).then(finish, e => this.emitError(e))
 }
 
 function flushHandler (done) {
@@ -43,6 +48,7 @@ export class TransformStream extends Transform implements IBluestream {
   private handlingErrors: boolean
   private queue: Set<Promise<any>>
   private streamEnd
+  private doneThisTick: boolean
   // tslint:disable-next-line
   private _flush: (done: (err, data) => void) => void;
 
@@ -83,6 +89,7 @@ export class TransformStream extends Transform implements IBluestream {
     this.handlingErrors = false
     this.concurrent = opts.concurrent
     this.queue = new Set()
+    this.doneThisTick = false
     this.once('finish', () => this.streamEnd.resolve())
   }