Turn client_order_index into a string

[DoganM95]

Currently the order creation requires a nonce of type integer for client_order_index and has the constraint of

nonce should not be larger than 281474976710655

This is fine for normal trading if you use a unix timestamp as nonce, but for HFT where multiple orders can be placed at the same exact time, it breaks.
Turning the nonce into a string would allow using a hash of the whole request object, preventing collisions completely.

This would also solve any issue with HFT related nonce generation.