Default constructor for `MapRef`

[armanbilge]

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We currently have many MapRef implementations, based on Ref[Map], sharding, ConcurrentHashMap, etc. We should have an apply constructor that chooses a reasonable default implementation, probably ConcurrentHashMap.

[yisraelU]

@armanbilge I'd like to pick this up

[armanbilge]

@BalmungSan raised a good point that this default constructor should only require a Concurrent constraint. We can pattern-match for Async to use ConcurrentHashMap implementation, otherwise maybe we can fallback to the sharded Ref[Map] with the number of shards determined by the number of processors.

[Jasper-M]

While we're at it, a nicer API for defaultedMapRef would also be welcome. E.g. a constructor that directly takes a default value, or an instance method on trait MapRef. MapRefOptionOps is probably a good place for it too.

[armanbilge]

MapRefOptionOps is probably a good place for it too.

Yes, I think this would be my vote. I think should be easy to chain with the constructor as well so we won't need a separate constructor?