fix: clear the polling timeout when stopped

[nicholasgriffintn]

Resolves: #234

Follow up from: https://github.com/bbc/sqs-consumer/pull/298, without the abort functionality, just the timeout.

Changes

• Define the timeout so that we can control it
• Clear the timeout when .stop() is called
• Before setting the timeout, check that it does not already exist, if it does, clear it

The issue

As consumer does not clear the timeout, when calling start, stop and then start:

  // eslint-disable-next-line no-console
  console.log('STARTING SQS CONSUMER');
  consumer.start();
  // eslint-disable-next-line no-console
  console.log('STOPPING SQS CONSUMER');
  consumer.stop();
  // eslint-disable-next-line no-console
  console.log('STARTING SQS CONSUMER');
  consumer.start();

Consumer effectively calls everything twice, as the timeout is still running from the original consumer instance, as shown below (two Polling for messages logs):

STARTING SQS CONSUMER
  sqs-consumer Starting consumer +0ms
  sqs-consumer Polling for messages +0ms
STOPPING SQS CONSUMER
  sqs-consumer Stopping consumer +2ms
RECEIVED SQS STOPPED:
STARTING SQS CONSUMER
  sqs-consumer Starting consumer +0ms
  sqs-consumer Polling for messages +0ms
EXPRESS APP LISTENING ON: http://localhost:3026
  sqs-consumer Received SQS response +20s
  sqs-consumer {
  sqs-consumer   '$metadata': {
  sqs-consumer     httpStatusCode: 200,
  sqs-consumer     requestId: undefined,
  sqs-consumer     extendedRequestId: undefined,
  sqs-consumer     cfId: undefined,
  sqs-consumer     attempts: 1,
  sqs-consumer     totalRetryDelay: 0
  sqs-consumer   },
  sqs-consumer   Messages: undefined
  sqs-consumer } +1ms
RECEIVED SQS EMPTY:
  sqs-consumer Received SQS response +2ms
  sqs-consumer {
  sqs-consumer   '$metadata': {
  sqs-consumer     httpStatusCode: 200,
  sqs-consumer     requestId: undefined,
  sqs-consumer     extendedRequestId: undefined,
  sqs-consumer     cfId: undefined,
  sqs-consumer     attempts: 1,
  sqs-consumer     totalRetryDelay: 0
  sqs-consumer   },
  sqs-consumer   Messages: undefined
  sqs-consumer } +0ms
RECEIVED SQS EMPTY:
  sqs-consumer Polling for messages +1ms
  sqs-consumer Polling for messages +2ms