Use common structs from hermez-node as much as possible

[arnaubennassar]

Avoid having custom structs just for the sake of marshaling JSONs. Note that this is needed in some cases if the hermez-node structs don't have the marshalers implemented.

• AtomicGroup
• PoolL2Tx
• AccountCreationAuth depends on hermeznetwork/hermez-node#1030

[jeffprestes]

Agreed 100%.
However, we need to adjust some items in hermez-node for that @arnaubennassar .
Please allow me to give an example. You've created a method called MarshalJSON for PoolTxAPI at db/l2db/views.go and this method, instead of return a defined type, it returns a interface{}
Could you please adjust this in hermez-node and later leave a comment here to allows us to use the public hermez-node type?

[arnaubennassar]

This issue should be implemented once this one is closed hermeznetwork/hermez-node#1030