Handler shutdown is not graceful: it does not wait all log to be sent

[vietk]

When the log handler is being closed, the underlying HttpEventCollectorSender is closed as well.
However by default, the sender is only shutting down the executor service. It has two impacts :

1. Queued logs will be not sent
2. Some error log may arise when queued logs like this one Error while sending events to Splunk HEC: executor rejected java.io.InterruptedIOException: executor rejected

It's a valid use case that an application could try to send its logs during a certain period of time to ensure persistance in Splunk

Implementation idea :

• We could configure and pass the to TimeoutSettings structure to the HttpEventCollectorSender constructor with a non-0 default termination timemout
• Pay attention the same structure is used in flushHttpClient that is called on every flush (i.e the flush become synchronous until timeout is reached)