Add "safe" mode with no data loss

[Victor Porton]

It is very frustrating if a user destroys a database and needs to create it anew from an old backup, or worse there is no backup.

So, please introduce "safe mode" (per database, per table, per connection, per database per connection, per table per connection) in which it's impossible to destroy data since a cetain "savepoint".

Suppose, I create a savepoint for a table. After this I will be unable (until I remove the save point) to:

- delete this table
- delete columns
- change exisitng columns

but will be able:

- create new columns
- change values of columns created after the savepoint
- rename columns
- add/remove indexes, constraints and foreign keys

We could also introduce "more safe mode" in which it's impossible to remove a time-consuming to create index.

Ideally, we should be able to have multiple savepoints ("not to damage the production data", "not to damage a costly cache table data") at the same time.