Use `isInvalidOffset` to check whether the committed offset is known
Use `isInvalidOffset` to check whether the committed offset is known

In some cases it seems possible that the committed offset isn't '-1', but rather undefined:
~~~~
kafkajs.Runner - Error when calling eachBatch ({"topic":"carrier-events","partition":0,"offset":"13435","stack":"TypeError: Cannot read property 'low' of undefined\n    at Function.fromValue (/app/node_modules/long/src/long.js:286:25)\n    at subtractOffsets (/app/node_modules/@collaborne/kafkajs/src/consumer/offsetManager/index.js:110:44)\n    at subtractPartitionOffsets (/app/node_modules/@collaborne/kafkajs/src/consumer/offsetManager/index.js:114:7)\n    at /app/node_modules/@collaborne/kafkajs/src/consumer/offsetManager/index.js:120:44\n    at Array.map (<anonymous>)\n    at subtractTopicOffsets (/app/node_modules/@collaborne/kafkajs/src/consumer/offsetManager/index.js:120:27)\n    at Array.map (<anonymous>)\n    at OffsetManager.countResolvedOffsets (/app/node_modules/@collaborne/kafkajs/src/consumer/offsetManager/index.js:122:37)\n    at OffsetManager.commitOffsetsIfNecessary (/app/node_modules/@collaborne/kafkajs/src/consumer/offsetManager/index.js:189:12)\n    at ConsumerGroup.commitOffsetsIfNecessary (/app/node_modules/@collaborne/kafkajs/src/consumer/consumerGroup.js:295:30)\n    at commitOffsetsIfNecessary (/app/node_modules/@collaborne/kafkajs/src/consumer/runner.js:235:34)\n    at messageGenerator (/app/node_modules/@collaborne/event-stream-library/src/client/shared-consumer.ts:341:10)\n    at runMicrotasks (<anonymous>)\n    at runNextTicks (internal/process/task_queues.js:62:5)\n    at processImmediate (internal/timers.js:429:9)\n    at process.topLevelDomainCallback (domain.js:137:15)"})
~~~~

From what I can see this might be related to `seek` and `resolve` operations overlapping, which
is possible: `seek` is only possible when the consumer is running, and when the consumer is running
it will do resolving/committing. This is likely a bigger issue when the consumer group contains
many topics.

After reading the code for quite a while I think the fix here is trivial: Instead of checking
explicitly for `-1` we simply should use the helper function created for that purpose.

tulios/kafkajs,tulios/kafkajs,tulios/kafkajs,tulios/kafkajs