Detect non-existing keys for CRDTs [JIRA: CLIENTS-552] #536

There is a semantic difference between non-existing key and a default value may me important for downstream consumers. However, CRDT queries return default values: 0 for a counter and empty set for a set. My following examples are in Scala:

val fetchOp = new FetchCounter.Builder(location).build()
val c = client.execute(fetchOp).getDatatype // if I get 0, does the key exist in store?

For counters, there is a workaround that involves an additional query:

val fetchOp = new FetchValue.Builder(location).withOption(FetchValue.Option.HEAD, java.lang.Boolean.TRUE).build()
val res = client.execute(fetchOp) // res.isNotFound should equal(true). 
// Why isNotFound not supported by CRDTs?

For sets, there seem to be a hack that a context object should be null if set did not exist before (not sure if it's a valid way though).

The concept of non-existing key is important enough to support it explicitly (like with regular objects in the snippet above).