Enforce, document thread-safety of `CsvSchema`

[cowwoc]

Unless I'm mistaken,

jackson-dataformats-text/csv/src/main/java/com/fasterxml/jackson/dataformat/csv/CsvSchema.java

Line 890 in de19091

protected int _features = DEFAULT_ENCODING_FEATURES;

seems to be the only mutable field in CsvSchema and it could be made final as well.

If CsvSchema is immutable and/or thread-safe, it would be helpful for the documentation to state as much. You could also convert _features to a final field which would make this more clear on an implementation-level. Instead of setting _features multiple times, you could move this logic into the constructors and only assign it once.

[cowtowncoder]

Yes, making CsvSchema immutable would be useful -- I thought it already was TBH -- and documenting it as such as well.

[cowtowncoder]

Yes, so actually CsvSchema is immutable for all practical purposes... unless someone decided to sub-class it. Not sure why _features was declared that way.

I will change it to be final and add some verbiage in Javadoc.