When WL#2775 was developed we decided to follow the pattern used by
log tables where direct updates are forbidden. The idea was to later
on provide users with the necessary commands to execute maintenance
tasks against the replication tables such as changing the engine in
use or dumping data. 

However, after pushing the WL#2775, we have realized that this would
unnecessarily duplicate code, i.e. functionality.

So, in this patch, we revert the code that follows the behavior
adopted by log tables and allow any sort of statement to be executed
against the replication tables.

This change causes a side effect that it is worth noticing. It has
made the relay-log-info file or not-transactional tables being updated
optimistically prior to committing an user transaction that is being
executed. This may lead to data loss if a crash happens as the user
transaction can be rolled back but it won't be re-applied automatically
upon restart.

This patch also makes sure that the server fails gracefully if some
unexpected changes are executed against the tables. For example, if
some columns are dropped.