[Protobuf] add a way to switch between RLP and Protobuf for substateDB decoding

[rpl-ffl]

Squashed from #86

Add a builder function SetDecoder(encoding string) to substateDB and SubstateDB interface so that a decoder type can be selected.
A substateDB can decode using decodeRlp or decodeProtobuf. It defaults to decodeRlp to preserve original functionality.

Example usage:

db := (&substateDB{...})          // initializing db as normal, this defaults to RLP
         .SetDecoder(<encoding>)  // at end of init (or before decoding) to switch between RLP/Protobuf

Type of change

• New feature (non-breaking change which adds functionality)

[cabrador]

Please add unit tests.

[rpl-ffl]

Deprecated in favor of #88, as discussed.