Data race in ConcurrentCharLoader.java

[spalac24]

Hi,

I believe I found a data race in ConcurrentCharLoader.java , which can be triggered by default normal use of the CsvParser library.

Here's a sample minimal code that triggers the issue: https://gist.github.com/spalac24/0256b40b1d6652fdb30e91624f7b5ed0

In short, that code causes stopReading() to be called both from https://github.com/uniVocity/univocity-parsers/blob/master/src/main/java/com/univocity/parsers/common/input/concurrent/ConcurrentCharLoader.java#L101 (which runs in its own thread by default) and https://github.com/uniVocity/univocity-parsers/blob/master/src/main/java/com/univocity/parsers/common/input/concurrent/ConcurrentCharInputReader.java#L85 concurrently, but stopReading() is not thread safe.

This does not reliably cause an exception to be thrown because repeatedly closing a Reader is fine as per the Java spec, but I believe it should still be addressed.