first cut: Cassandra flows with support for conditional writes

[leviramsey]

"Conditional" write flows result in Either[T, AsyncResultSet]: the Left[T] case is for when the write was applied, the Right[AsyncResultSet] case is when the write wasn't applied (e.g. in an INSERT ... IF NOT EXISTS there was already a row). Per the Cassandra docs around wasApplied, if the query is an unconditional write, this will always be a Left (for those, the conditional version of the flow is effectively just a less efficient version (extra wrapping/unwrapping) of the existing flow). I'm open to flipping the Either, but my sense is that this doesn't perfectly map onto the usual right-biasing conventions (the more interesting result is the AsyncResultSet IMO, even though that could be thought of as the failure).

The Java API is a work in progress: this feels like how a sum type would be encoded in a lightweight way in Java and I don't know if it makes sense to expose functional combinators: since the typical next stage in the Scala API would be a fold, I think Java code like:

if (element.wasApplied()) {
    // do stuff with element.getContents()
} else {
    // do stuff with element.getResultSet()
}

Perhaps the fold could be incorporated into the stage by requiring an AsyncResultSet => T, but that actually seems like a fairly niche use case