Update base rate for enabled swaps from uniswap each block

[palango]

Sub-issue for #6506

Description

If token swaps are enabled, the node 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
• Only query the currently enables token pairs to lower load on the ethereum nodes.

[karlb]

We could also fetch the rate during the mediation. This would vastly reduce the number of calls and should be still acceptable from a latency perspective.