Reconsider get/getCounter and scan/scanCounters

[plaflamme]

Having get/getCounter makes interacting with cockroach a lot easier since you normally know what type of value you're expecting (i.e.: bytes or counter).

That said, calling one version on the wrong type currently succeeds and returns a None. That's probably undesirable.

Similarly, scan and scanCounters currently skip values of the wrong type. If a client needs to scan both, this is inefficient.

This can be solved by introducing a Value type that has a Bytes and Counter implementation. Consider this signature:

sealed trait Value
case class BytesValue(bytes: Bytes) extends Value
case class CounterValue(value: Long) extends Value

class Client {
  def get(key: Bytes): Future[Option[Value]]
  def getBytes(key: Bytes): Future[Option[Bytes]] // fails when reading a counter
  def getCounter(key: Bytes): Future[Option[Long]] // fails when reading a counter

  def scan(from: Bytes, to: Bytes): Future[Spool[(Bytes, Value)]]
  def scanByteValues(from: Bytes, to: Bytes): Future[Spool[(Bytes, Bytes)]] // skips counters
  def scanCounterValues(from: Bytes, to: Bytes): Future[Spool[(Bytes, Long)]] // skips bytes
}

Arguably, we could use Either[Bytes, Long] but would make pattern matching less explicit (case Right(counter) vs. case CounterValue(counter))