syncbase: improve comments about concurrency

- Updates client-facing model.go's to mention what happens
  with Scan/Exec and concurrent writes
- Updates storage engine model.go to document concurrency
  semantics
- Other misc related comment tweaks

Change-Id: Iad5f50334ae942ffab06170caf5ec8c9c4b270bf
1 file changed
tree: 87041a95598ce0af88c240097041c6a3960343f8
  1. services/