This PR cleans up the configurations for pruner and snapshotter by passing a config struct.

And also, this PR disables the snapshot background generation if the chain is opened in "read-only" mode. The read-only mode is necessary in some cases. For example, we have a list of commands to open the etheruem node in "read-only" mode, like export-chain. In these cases, the snapshot background generation is non expected and should be banned explicitly.