Break Stream in two, rewrite core part of Stream, in prep for new shards

[knighton]

Having parted the Streams:

• The "upper" StreamCore is just all the args that are shared by all its Shards -- Shards then contain a reference to the owning StreamCore
• The "lower" Stream (subclass) is everything else, nobody knows or cares StreamCore dirty circular import defeat trick exists
• Also fix bug in generating deterministic temp path given remote + split when have remote but no local

Result of init sequence (more args to come):

StreamCore:
    remote: Optional[str]
    local: str
    split: Optional[str]
    download_retry: int
    download_timeout: float
    hash_algos: List[str]
    keep_old_phases: str
    safe_keep_old_phases: str

Stream:
    ... above plus ...
    proportion: Optional[float]
    repeat: Optional[float]
    choose: Optional[int]