Fix broken logic when appending in async

[python36]

Test code:

import nmqtt
import asyncdispatch

let localCtx = newMqttCtx("test_async")
localCtx.setHost("localhost", 1883)
waitFor localCtx.start()

proc test(ind: string) {.async.} =
  for i in 1..2:
    await localCtx.publish("test/" & ind & "/" & $i, "test", 0, false)
    await sleepAsync 1

proc main() {.async.} =
  asyncCheck test("1")
  asyncCheck test("2")

asyncCheck main()

runForever()

Run listener:

mosquitto_sub -p 1883 -t "test/#" -v

Result:

test/1/1 test
test/2/1 test
test/1/2 test

Skipped test/2/2 test!

---

Problem (nmqtt.nim):
Since the table is copied before the loop, if a new Work is added while the loop is being processed, it will be ignored.

proc work(ctx: MqttCtx) {.async.} =

# ...

    for msgId, work in workQueue:

      #when defined(broker):
      if work.typ in [ConnAck, SubAck, UnsubAck, PingResp]:
        if await ctx.sendWork(work):
          delWork.add msgId
          continue

      if work.wk == PubWork and work.state == WorkNew:
        if work.typ == Publish and work.qos == 0:
          if await ctx.sendWork(work): delWork.add msgId # <--- at this point

        elif work.typ == PubAck and work.qos == 1:
          if await ctx.sendWork(work): delWork.add msgId # <--- at this point

        elif work.typ == PubComp and work.qos == 2:
          if await ctx.sendWork(work): delWork.add msgId # <--- at this point

        else:
          if await ctx.sendWork(work): work.state = WorkSent # <--- at this point

      #when not defined(broker):
      elif work.wk == SubWork and work.state == WorkNew:
        if work.typ == Subscribe:
          if await ctx.sendWork(work): work.state = WorkSent # <--- at this point

        elif work.typ == Unsubscribe:
          if await ctx.sendWork(work): # <--- at this point
            work.state = WorkSent
            ctx.pubCallbacks.del work.topic

# ...

---

Solution: After looping through the table, check whether new Works have been added. Result after changes:

test/1/1 test
test/2/1 test
test/1/2 test
test/2/2 test

test/2/2 test not skipped!