Fix/dlq timestamp

[robbavey]

Stop timestamp overriding committed offsets

Change behavior to only respect the 'startTimestamp' flag when no offsets have been committed.
Adds more tests.

Fixes #6

[jakelandis]

minor - can you add the @Override annotation ?

[robbavey]

Will do

[jakelandis]

minor/style - since close is throws IOException this can implement Closeable instead of AutoClosable, which (imo) is a bit more idiomatic

[robbavey]

I don't have strong feelings either way - I added the AutoCloseable interface to allow the use of try.. with resources to help avoid resource leaks/clean up tests. My understanding was the AutoCloseable was more used for code targeting JDK7+, but I guess you are right in that the existing close() throws IOException implies Closeable. (That also means that I need to fix up the method to make it idempotent, which it should have been anyway... ;))

[ph]

Its this PR related to fix the CI test on master and the 6.x branch?

https://travis-ci.org/logstash-plugins/logstash-input-dead_letter_queue

dead_letter_queue/src/test/java/org/logstash/input/DeadLetterQueueInputPluginTests.java:109: error: cannot find symbol
                targetDateString = entry.getEntryTime().toIso8601();
                                                       ^
  symbol:   method toIso8601()
  location: class Timestamp
1 error
:compileTestJava FAILED
:compileTestJava (Thread[main,5,main]) completed. Took 1.579 secs.

[robbavey]

@ph - the reason the tests are failing on 6.x and master is elastic/logstash#8128 (elastic/logstash@d121c58), which removes the toIso8601 method from timestamp.

This PR does remove that method, but a less invasive PR should probably go in between. I'll put it up shortly

[ph]

Thanks @robbavey !