Fix vAccount action payload serialization to work with big payloads

[MrToph]

The existing code to serialize vAccount actions only works with payloads that don't exceed a length of 255 bytes as it always uses a single byte for the payload size. Although this works fine in your vaccountconsumer test due to the small payload, I think it's a good idea to change it there as well because I'm sure many developers, including me, will use that code as their starting point until there's documentation.

Changelog

• Change the serialization of data to match the serialization in EOSIO's CPP datastream for std::vector<char> which uses a varuint32_t for the size.
• Serialize an empty std::vector<permission_level> authorization array instead of a one-element array with an empty permission level. This saves some bytes for every vAccount action.
• Add comments documenting what's going on.

[NatPDeveloper]

Thank you @MrToph for this! I will get this code setup in our staging environment to be merged.