Close not working as expected in a lambda

[cipriancaba]

Describe the bug
When running logzio in an AWS lambda I did the logger.close() at the end of the execution as I was instructed.

However, if you close the logger and the lambda gets reused, the logger is closed and won't ship any more logs and will error with Attempt to write logs with no transports, which can increase memory usage

To Reproduce
Steps to reproduce the behavior:

1. Run a lambda and initialize the logger
2. Run close() at the end of the execution as instructed per docs
3. Next warm execution of the lambda will fail logging

Expected behavior
Replace close with a flush that will instruct flushing the current batch of logs

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