Support Relative Time Expressions To Be Used in Between Expression

[mrblueblue]

declare type NowExpression = {|
  type: "now"
|}

declare type RelativeTimeExpression = {|
  type: "relative",
  interval: "minute" | "hour" |  "day" | "week" | "month" | "quarter" | "year",
  step: number
|}

[nytai]

This is an interesting question. Aside from providing a sensible api, there is the question of where the timestamps are generated. In immerse currently (due to mapd-core not supporting datediff functions) the timestamps are generated before the query is issued. It's likely that we would want to offload the relative computations to the backend, via datediff functions, however if there is a caching layer this might be an issue (as queries would be the same).