KeyValue atomic delete and purge methods.

[davidmcote]

nats-io/nats-architecture-and-design#271
#1091

What's in this patch?

1. New public methods in KeyValue for delete and purge with optimistic concurrency support:

void delete(String key, long expectedRevision) throws IOException, JetStreamApiException;
void purge(String key, long expectedRevision) throws IOException, JetStreamApiException;

2. New unit test testAtomicDeleteAtomicPurge with positive and negative cases.

[davidmcote]

(Brain fart - I let github rebase a signed commit and it predictably became unverified. I re-pushed the signed commit.)

[scottf]

LGTM