Renaming for testnet release

[ptagl]

The following RPC commands have been renamed:

• create_sidechain => sc_create
• sc_create => dep_sc_create
• send_to_sidechain => sc_send
• sc_send => dep_sc_send
• request_transfer_from_sidechain => sc_request_transfer
• send_certificate => sc_send_certificate

Additionally, the following RPC JSON fields have been renamed:

• "withdrawalEpochLength"
• "backward transfer" => "backwardTransfer"

This PR closes #158 #159 #160 #166.