Allow maxRetries to be configurable

This issue was recently fixed and I stress tested it myself:
#1551

It introduced the maxRetries variable in FieldtypeMulti.php, which is hardcoded to 3.

Ideally, I'd like to be able to modify it to 5 for example. Can this be made into a config variable so it can be easily overridden?

Actually it seems Adrian Jones already proposed it (along with a few other options) here:
#2038 (comment)

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