Rename /transfers/* endpoint to /payments/*

[palango]

Problem Definition

In Tenerife we decided that a payment is The process of sending tokens from one account to another. May be composed of multiple transfers (Direct or HTL). A payment goes from Initiator to Target.

In #1949 we add a view into the payment history. This issue is to rename the current '/transfers/<hexaddress:token_address>/<hexaddress:target_address>' entpoint.

ToDo:

• Rename POST '/transfers/<hexaddress:token_address>/<hexaddress:target_address>' to `POST '/payments/hexaddress:token_address/hexaddress:target_address'
• Update the docs
• Update the WebUI

[LefterisJP]

In Tenerife we decided that

Please do not mention that as a reason to do anything. I'd rather see links to documentation. I recall that conversation but it's not well documented so I at least added that definition to the glossary: #2095
Note: It's also in the raiden network spec repo.