Log events in a single PutLogEvents request must be in chronological order

[KiNgMaR]

Re-creating a new entry for the problem explained in #32, because the fix there had to be reverted and the exception is still occurring. I'm also investigating libraries for other languages for potential fixes.

Timestamps in my case look e.g. like this:

1568894292321.3
1568894292321.3
1568894292322.5
1568894292322.5
1568894292322.5
1568894292322.5
1568894292323.6
1568894292323.6
1568894292323.6
1568894292323.6
1568894292324.7
1568894292324.7
1568894292324.7

[KiNgMaR]

Two findings so far:

• AWS actually expects an integer, not a float, for the number of milliseconds. Ref: https://docs.aws.amazon.com/AmazonCloudWatchLogs/latest/APIReference/API_InputLogEvent.html
• in my case, the events are actually not sorted chronologically. They are coming "unsorted" from Monolog, probably because I use the same CloudWatch handler for multiple log targets in Symfony.
• based on this, I assume that duplicate timestamps should not present an issue as long as the entries are well-sorted.

Possible solution, seeing that this might be an AWS-specific problem:

• sort entries before submission. This might have some performance drawbacks though, so that would need to be explored.

Workarounds:

• still investigating

[KiNgMaR]

I wasn't able to figure out exactly why Symfony (or Monolog) would submit entries not already sorted chronologically, but in the end it does not matter: the requirement for submitting entries chronologically sorted seems to be a peculiarity of AWS CloudWatch logs, and therefore treating that issue within a CloudWatch-specific library seems appropriate.

I opened a PR with sort logic implemented which (at least for me) fixes the problem. Feedback appreciated :)