fftokens: revisit ERC20 tokens symbol

[shorsher]

[awrichar]

Currently FireFly takes in a name and symbol as part of the "create token pool" request, and passes these both to the token connector.

Need to give some thought as to how we handle 3 different scenarios:

1. connector doesn't care about name/symbol (ie ERC1155)
2. connector deploys a contract itself and takes name/symbol as input (ie ERC20 factory)
3. connector uses an existing contract and supplies name/symbol as output (ie pre-deployed ERC20)

[awrichar]

Resolved by #606 and hyperledger/firefly-tokens-erc20-erc721#31