execution: use read-only tx in LocalState

[canepat]

Both LocalState and RemoteState are implementations of the State interface aimed at providing support for call execution, i.e. transaction re-execution or simulation required by many RPC end-points.

As such, LocalState must use a read-only database transaction because:

1. it doesn't need any read-write support (no persistent change is produced)
2. can be instantiated multiple times concurrently to offer parallel call execution