The word "source" implies reading, while in fact access is read/write.
"datastore" avoids that misconception. Writing it in one word emphasizes
that it is single entity.

While renaming, also remove references to explicit --*-property
parameters. The only necessary use today is "--sync-property ?"
and "--datastore-property ?".

--datastore-property was used instead of the short --store-property
because "store" might be mistaken for the verb. It doesn't matter
that it is longer because it doesn't get typed often.

--source-property must remain valid for backward compatility.

As many user-visible instances of "source" as possible got replaced in
text strings by the newer term "datastore". Debug messages were left
unchanged unless some regex happened to match it.

The source code will continue to use the old variable and class names
based on "source".

Various documentation enhancements:
  Better explain what local sync is and how it involves two sync
  confi...