[SPARK-19813] maxFilesPerTrigger combo latestFirst may miss old files in...

[SPARK-19813] maxFilesPerTrigger combo latestFirst may miss old files in combination with maxFileAge in FileStreamSource

## What changes were proposed in this pull request?

**The Problem**
There is a file stream source option called maxFileAge which limits how old the files can be, relative the latest file that has been seen. This is used to limit the files that need to be remembered as "processed". Files older than the latest processed files are ignored. This values is by default 7 days.
This causes a problem when both
latestFirst = true
maxFilesPerTrigger > total files to be processed.
Here is what happens in all combinations
1) latestFirst = false - Since files are processed in order, there wont be any unprocessed file older than the latest processed file. All files will be processed.
2) latestFirst = true AND maxFilesPerTrigger is not set - The maxFileAge thresholding mechanism takes one batch initialize. If maxFilesPerTrigger is not, then all old files get processed in the first batch, and so no file is left behind.
3) latestFirst = true AND maxFilesPerTrigger is set to X - The first batch process the latest X files. That sets the threshold latest file - maxFileAge, so files older than this threshold will never be considered for processing.
The bug is with case 3.

**The Solution**

Ignore `maxFileAge` when both `maxFilesPerTrigger` and `latestFirst` are set.

## How was this patch tested?

Regression test in `FileStreamSourceSuite`

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #17153 from brkyvz/maxFileAge.