Disallow DML operations on connections with CURRENT_SCN set

We should make a connection read-only if CURRENT_SCN is set. It's really a bad idea to go back in time and update data and it won't work with secondary indexing, potentially leading to your index and table getting out of sync.

For testing purposes, where we need to control the timestamp, we should rely on the EnvironmentEdgeManager instead to control the current time.