Create JsonReadFeature to move JSON-specific JsonParser.Features to

[cowtowncoder]

Currently JsonParser serves dual role of being both base parser/decoder API and its JSON-backed implementation. Ideally we would split it into 2 parts (similar to how JsonFactory was changed to extend TokenStreamFactory, new API, and become just implementation), but due to wide-spread usage in mapper api this is impractical.

But one aspect we can improve is to split JsonParser.Feature entries: most of them are strictly JSON-specific, and only couple have general applicability.
While we could half-ass this change, too, we probably need to do full change in 3 distinct parts:

1. New JSON-specific feature, like JsonReadFeature (for most entries). For 2.10, has to duplicate entries; for 3.x can actually move. 2.10 probably has to map these to JsonParser.Feature
2. New general-purpose feature, maybe StreamReadFeature (for couple of general purpose ones). For 2.10, could simply map to existing JsonParser.Features (implementation-wise); for 3.x, probably want to reverse.
3. JsonParser.Feature itself; entries to be deprecated (perhaps even in 2.10?).

It also seems likely that although we can forward-proof API to a degree with 2.10, implementation probably differs so that:

• 2.10 will map new features into old flags, and basically not add much internally aside from delegation
• 3.0 should map deprecated JsonParser.Feature shared flags (since they are more widely used), but can just drop JSON-specific ones as delegation might get tricky. This is backwards-incompatible but probably better than attempts at hybrid; most common case is probably that of comment-enabling.