building computeKey cache on first `fcu` call
https://github.com/status-im/nimbus-eth1/blob/0b93236d1b6d7436e92ed39d44ac25008326a0f6/nimbus/config.nim#L531-L535

The default mode should be with validation, will also smooth out the UX from `import` to `syncer`. As with validation the triggering of the computeKey cache won't take place with the first `fcu` call which takes hours and results in the `fcu` call being timed out

Suggestion: `defaultValue: false`