Avoid unnecessary fee_rate updates by checking last_best_block_hash

[moisesPompilio]

When using BitcoindChainSource, the node was updating the fee_rate every 2 seconds, even if no new block had arrived. This behavior did not occur with other chain sources.

This change introduces a check against the last_best_block_hash before updating the fee rate in the periodic loop. If the current block hash is the same as the last known one, the update is skipped. The fee rate is only refreshed when a new block is detected, reducing redundant processing.

Note that this only affects the periodic update loop. Direct calls to update the fee_rate are not impacted

Close #520

[tnull]

Hmm, I think updating once per tip currently makes sense as Core estimates based on recently-mined transactions anyways. We might want to change that once they offer mempool-based fee estimation, which should be much more exact.

Please undraft!

[moisesPompilio]

Ok, it was draft while I checked if a similar optimization was needed for Electrum/Esplora sources, but since they don’t have excessive updates I think no change is needed there for now.