add idempotency error "duplicate request"

[mtrezza]

Adds new Parse Error 159: DUPLICATE_REQUEST.

This PR completes the server side implementation of parse-community/parse-server#6748.

[codecov]

Codecov Report

Merging #1189 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1189   +/-   ##
=======================================
  Coverage   92.34%   92.34%           
=======================================
  Files          54       54           
  Lines        5293     5294    +1     
  Branches     1191     1191           
=======================================
+ Hits         4888     4889    +1     
  Misses        405      405           

Impacted Files	Coverage Δ
src/ParseError.js	100.00% <100.00%> (ø)

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[SebC99]

I love the idea, especially for "long" requests. Not sure how the clients will have to be adapted to silently cope with this error, so we have to mark it as breaking change:
Before this, there was no error, just duplicates results... and now it will result in an error for all the duplicate calls.

[mtrezza]

I love the idea, especially for "long" requests. Not sure how the clients will have to be adapted to silently cope with this error, so we have to mark it as breaking change: Before this, there was no error, just duplicates results... and now it will result in an error for all the duplicate calls.

@SebC99 This is not a breaking change. This PR merely adds an error code and occupies the currently unused error code 159 for that matter. This is part of an experimental feature that can be turned on / off. Even when it matures out of experimental status it should not be on by default due to the overhead it incurs, so I assume at this point that this will never be a breaking change.

[SebC99]

👌

[dplewis]

LGTM!