QueryTranslator: Implemented predictive index support

[snej]

When implementing the QueryTranslator, I screwed up and forgot to implement index-based predictive queries. Early on in development I commented out groups of tests I hadn't gotten working yet, and somehow I forgot to take out the #ifdefs for the predictive-index tests, which is why I didn't notice the problem.

This PR remedies that. Unfortunately it required more code change than I'd like, because predictive indexes work differently than vector or FTS, in that the query works with or without an index. That means when parsing a PREDICTION() call the parser has to know whether an index exists, which involves calling out to the QueryTranslator's delegate. That's where the ParseDelegate comes from.

Smaller changes:

• Factored out some common code into IndexedNode so the new code can use it
• Renamed "indexExpressionJSON" to "indexID" because the name was misleading, as it isn't always JSON.

[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	66.07
functions	78.73
instantiations	33.19
lines	77.71
regions	73.34