Update base rate from uniswap each block

[palango]

Sub-issue for raiden-network/raiden#6506

Description

The PFS needs up-to-date information about swapping rates. Therefore it needs to update the base rates each time a new block is received.

Tasks

• Implement efficient (if possible batched) request to fetch latest base rates
  • If possible design an interface that make using different oracles later easy
  • Maybe Update base rate for enabled swaps from uniswap each block  raiden#6513 is reusable
• Only query the currently enables token pairs to lower load on the ethereum nodes.

Refernces

• Update base rate for enabled swaps from uniswap each block  raiden#6513