retire s3n

S3A is now stable and performant, s3n is unmaintained.

Moving from s3n to s3a is straightforward: change your key names.

I propose

1. remove s3n main and test source, except for a reference in the docs, "gone"
2. drop the jets3t JAR
3. migrate any open s3n JIRAs which apply to s3a too over to being about s3a
4. close any other remaining s3n JIRAs

short term, branch-2 and 3.0 alpha should issue a deprecation warning the first time s3n is used in a JVM