Support MongoDB's findAndModify

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Parse does not natively support a way to issue a query and in the same operation write to the found documents. This is a concept used to uniquely process documents in an environment with concurrent operations.

Feature / Enhancement Description

Add a method set or update to Parse.Query that if set executes Parse.Query.first as a findAndModify in MongoDB.

Example Use Case

1. Request A is a findAndModify request for documents that are without a "being processed" flag, which returns one document which will be flagged as being processed immediately.
2. Request B is the same as request A and has been issued at the time as request A. No document is returned, which ensures the same document is not being processed twice.

Alternatives / Workarounds

• Use transactions, which may have an unnecessary performance impact
• Directly access the DB adapter, see https://github.com/parse-community/parse-server/issues/4258
• Maintain a custom DB connection, directly interacting with the DB driver

Start by tracing Parse.Query.first and the MongoDB database adapter, then review the direct-adapter workaround in issue 4258. Define the API behavior for an atomic find-and-modify operation and verify it with concurrent requests: one request should claim a matching document and the other should return none.