ChainSelection could pass around a ExtLedgerState instead of an STM action for it

[jasagredo]

As noted by Alex here, it seems reasonable that we can pass around an ExtLedgerState because the ChainSelection is the only component that can alter that one, so the STM action will always return the same value.

However in order not to risk the implementation, we followed what was already there. Lets try it after the merge.