Minimize db query

[Marco129]

#896 do exist when allowClientClassCreation = true.

Using loadSchema and hasClass together will call Schema reloadData for three times, which is redundant in our case.
Changed to use collectionExists from db adapter.

Suppose user(admin) will not always update the schema if disabled allowClientClassCreation, we may consider cache the schema object in the future.

[codecov-io]

Current coverage is 90.19%

Merging #919 into master will decrease coverage by -0.83% as of a2d0b25

@@            master    #919   diff @@
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  Files           72      72       
  Stmts         4165    4163     -2
  Branches       862     862       
  Methods          0       0       
======================================
- Hit           3791    3755    -36
  Partial         10      10       
- Missed         364     398    +34

Review entire Coverage Diff as of a2d0b25

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[drew-gross]

Awesome!

[steven-supersolid]

I had allowClientClassCreation = [default value] when I ran the tests, which I think is true. Also had the useMasterKey: true in the query/save, so am not sure if the code path in the PR would be executed in that case as it checks if allowClientClassCreation === false?

Also, do we need to check allowClientClassCreation in RestQuery.js

I'll retest with allowClientClassCreation false and with this fix when there is a new version available.