Consider moving to jackson stream parsing

[karthikrg]

Its a lot faster than GSON, consumes lesser memory and does not use reflection. The generated code is a bit more verbose, but the tradeoff is worth it.

[jpbetz]

Pegasus uses streaming Jackson for the main parser, which Scala uses. But I agree, we should use streaming Jackson for android as well and eliminate our use of GSON. I'll keep this open.