fix(pluto): Handle write operations inside FlatMap with Future

[coveloper]

Description:

Identus example app consistently throws a recursive save error when calling .sendMessage() as described in #183

Wrapping these .flatMap write operations in a Future seems to fix it in my example.
*note: If this is an acceptable fix, we might need to add another similar Future inside addMessages()'s .flatMap

Alternatives Considered (optional):

I had tried adding guard self.hasChanges else { return } on line 12 of NSManagedObjectContext+Save.swift but that didn't seem to have any effect on this issue.

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