Consider renaming node.systemName

Discussion https://github.com/apple/swift-distributed-actors/pull/112#discussion_r326903899

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Neither are generated. Picking something like "shortName" might be ok here or "label" perhaps... We use them in first@12.3.4:999/user/path/actor and they are useful for human readable names fist/second or "us-west-1991" etc. We do NOT enforce them all have to be the same in a cluster -- unlike akka, where the name has to be the same for all nodes in the "same cluster", so there it is more of a "actor system name" since by system we like to talk about "all the nodes" 🤔 The same name thing is too restrictive and does not really help in real world, so we lifted that in swift. But we might also want to change the name then...

Short name is kind of what erlang calls them; there one can use "short names" or "full names -- which are IPs", though short names are only for local mode I believe.

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I'm thinking `shortName` or `name` or `label`?

Start with the linked pull-request discussion and locate the declaration and uses of `node.systemName`. Compare the proposed names `shortName`, `name`, and `label` against the existing terminology and usage. Done means a naming decision is reached and the selected name is applied consistently wherever this API is referenced.

Written by the indexing model from the issue text.