sql: allow transitive Lua <-> SQL transactions

This patch makes possible to start transaction in Lua and continue
operations in SQL as well, and vice versa. Previously, such transactions
result in assertion fault. To support them, it is required to hold deferred
foreign keys constraints as attributes of transaction, not particular VDBE.
Thus, deferred foreign keys counters have been completely removed from
VDBE and transfered to sql_txn struct. In its turn, if there is at least
one deferred foreign key violation, error will be raised alongside with
rollback - that is what ANSI SQL says. Note that in SQLite rollback
doesn't occur: transaction remains open untill explicit rollback or
resolving all FK violations.

Also, 'PRAGMA defer_foreign_keys' has been slightly changed: now it is not
automatically turned off after trasaction's rollback or commit. It
can be turned off by explicit PRAGMA statement only. It was made owing
to the fact that execution of PRAGMA statement occurs in auto-commit
mode, so it ends with COMMIT. Hence, it turns off right after turning on
(outside the transaction).

Closes #3237