Change access modifier for readAdvise in MMapDirectory

[navneet1v]

Description

Change access modifier for readAdvise in MMapDirectory. Made the access modifier protected so that classes which overrides MMapDirectory can use the readAdvise or if needed expose them.

I considered adding a getter for this parameter too, but went against it since the return type is pretty weird. I am open to adding a getting too. If we are happy with a getter like this:

public BiFunction<String, IOContext, Optional<ReadAdvice>> getReadAdvise() {
  return readAdvice;
}

or we can also expose a function like

public ReadAdvice getReadAdviseForFile(String filename, IOContext ioContext) {
    return readAdvice.apply(filename, ioContext);
}

This will ensure that consumers can validate if readAdvise is correctly set or not.

[navneet1v]

Will set change log soon