Expose flush() method on Logback and Log4j appenders to flush underlying HttpEventCollectorSender and OkHttpClient

[stevejagodzinski]

Build upon the previous partial solution to #152

Instead of stopping the appender at then end of a lambda function, just call flush() on the appender.

This way the appender can be re-used next time the lambda function is invoked.

The underlying httpClient is flushed (up to the configured timeout).

[stevejagodzinski]

The failing checks seem un-related to the changes in this MR. Perhaps the CI pipeline is unstable?

#210 aims to debug this pipeline failure.