Relative time queries (#4289)

* Add relative time queries

* Encode successful result

* Add integration test

* Add more error cases

* Remove unnecessary new Date

* Error when time has both 'in' and 'ago'

* naturalTimeToDate -> relativeTimeToDate

* Add $relativeTime operator

* Throw error if $relativeTime is invalid

* Add integration test for invalid relative time

* Exclude $exists query

* Only run integration tests on MongoDB

* Add it_only_db test helper
bd2ea87c1d/CONTRIBUTING.md (L23)

* Handle where val might be null or undefined

* Add integration test for multiple results

* Lowercase text before processing

* Always past if not future

* Precompute seconds multiplication

* Add shorthand for interval
hr, hrs
min, mins
sec, secs

* Throw error if $relativeTime is used with $exists, $ne, and $eq

* Improve coverage for relativeTimeToDate

* Add test for erroring on floating point units

* Remove unnecessary dropDatabase function

* Unit test $ne, $exists, $eq

* Verify field type

* Fix unit test for $exists
Unnest query object

spec/helper.js

@@ -409,6 +409,14 @@ global.it_exclude_dbs = excluded => {
   }
 }
 
+global.it_only_db = db => {
+  if (process.env.PARSE_SERVER_TEST_DB === db) {
+    return it;
+  } else {
+    return xit;
+  }
+};
+
 global.fit_exclude_dbs = excluded => {
   if (excluded.indexOf(process.env.PARSE_SERVER_TEST_DB) >= 0) {
     return xit;