SDK3: Sort out naming of expiration

Spotted by the ever-vigilant James Flather, we have

getOptions().withExpiration(true) 

But also

 collection.replace( "my-document",

 json, replaceOptions().expiry(found.expiration().get())

e.g. expiry vs expiration.

The RFC says expiration consistently, but James has raised it would be good to sync up with Sync Gateway here, that uses expiry.  Related discussion is here https://couchbase.slack.com/archives/CCX40TL9F/p1567701658038700