Add StreamReadFeature.CLEAR_CURRENT_TOKEN_ON_CLOSE (default: true)

[cowtowncoder]

(follow-up to #1438 )

Due to risks of regression, let's add a StreamReadFeature for determining whether JsonParser.close() should clear current token information, accessible via JsonParser.currentToken().

If enabled (default for 2.20), current token IS cleared (set to null); if disabled, it remains as it was at time of close().

Default planned to be true (clear current token) as that seems more intuitive choice; but note that existing (2.19) behavior what false will do.

Since this is for 2.20, we have still time to consider choice of default.
(for 3.0 it should probably be true, at any rate).