JsonTupleStore should have more user-friendly delimiter

[cjnolet]

The choice of the current delimiter (_$) was indirectly tied to the fact that jexl was very limited in the characters available for key names. Spark uses backticks around portions of keys that are nested, which makes it much more readable, however, it makes it a little more complicated for a user to have to type by hand.

I'd like to work this in to the 1.4.0 release.