fix: stateful currency network for ETH and BTC

[benjlevesque]

Description of the changes

The currency network was stateful, because the value stored in the map, which is a global variable, was changed.
This PR removes the use of the global variable to avoid mistake.

Example

stringToCurrency('ETH')
// { type: RequestLogicTypes.CURRENCY.ETH, value: 'ETH' }
stringToCurrency('ETH-rinkeby')
// { type: RequestLogicTypes.CURRENCY.ETH, value: 'ETH', network: 'rinkeby' }
stringToCurrency('ETH')
// { type: RequestLogicTypes.CURRENCY.ETH, value: 'ETH', network: 'rinkeby' }
//                                                       ^~~~~~~~~~~~~~~~~~

[coveralls]

Coverage increased (+0.005%) to 91.187% when pulling b6217d8 on fix-persisting-currency-state into 48efc12 on master.

[rittme]

Awesome job spotting this! It's a really tricky one :)