fee rate: connection / fallback logic

[esengulov]

This issue is a followup issue to horizontalsystems/unstoppable-wallet-ios#642

Transaction Fee Rates

• the first request should be made to HS IPFS node. If that doesn't respond in 10 seconds then request from ipfs.io (the timeout for it should be 60 seconds)
• of both of the above fail then use following fallbacks for each blockchain, as per fallback guidelines below.

Fee Fallback Guidelines:

Bitcoin: https://btc.horizontalsystems.xyz/apg/ .

The API documentation can be found here https://bcoin.io/api-docs/#estimatefee.
Priority parameters of this API (number of blocks ).

• high priority: 1
• medium: 6
• econom: 15

Responses are different from those you get from IPFS, it is in Bitcoin/kb. So in order to convert it format sat/bytes it needs to be multiplied by 100,000,000 and divided by 1024.

Bitcoin Cash and Dash:
Since both of those blockchains are currently loaded with 5% of their capacity, let's not provide fallback for these blockchains.

Ethereum:
Use Infura as fallback