Fixed C++ codegen ignoring multiple padding scalars per field.

Bug: 17373251, 17221979
Change-Id: Ib8b77835f0acd3290f0a5e7d0f683d9fdcbf7230
Tested: on Linux
Fixed flatc silently accepting non-scalars as default values.

Bug: 17304016
Change-Id: I4873f8ef32fbb2657f15fc53a2c8f767e10f2d96
Tested: on Linux
Setting a field twice in a JSON object now gives error.

Before, it would crash in FlatBufferBuilder.

Bug: 17357164
Change-Id: I6e6dbada5261745220345379eb53eb3eb113e8f8
Tested: on Linux.
Made sure GetRootAs..() functions are generated for all Java tables.

Previously they were only generated for the root_type, making it
impossible to use the other types in the file as the root of a buffer.

Bug: 17206174
Change-Id: Ie71bed42ac3b22dcceae6385cbd5846c37e5f1b8
Tested: on Linux