feat: StreamPosition type + fix: ditch Instant repr for timestamps

[sgbalogh]

Two changes:

• Introduces a StreamPosition type for capturing seqNum/timestamp pairs (in append ack, and checkTail responses)
• Removes Instant conversions when dealing with timestamp
  • This was assuming that all timestamps are epoch ms; this is not necessarily the case if a client supplies their own timestamp and opts to use a different convention, so I've moved to long