Allow disabling wallet background syncing

[tnull]

As Esplora syncing is heavily rate limited, we so far recommended to avoid manual syncing via sync_wallets and rather rely on the background syncing interval.

However, some users might have setups where they want to or have to rely on manually syncing the wallets. While they can currently bump the intervals to u64::MAX, we should consider supporting this explicitly by making the interval config settings Options and disabling background syncing of the wallets/fee rate cache if the corresponding value is disabled.