Touch txn a little bit to support multistatement transactions in
autocommit mode.

This patch allow to perform DML requests from space:on_replace()
triggers. Trigerred requests are processed as a part of the original
transaction and entire transaction is rolled back in case of error.
Please note that there is still no support for nested transactions,
therefore a call to box.begin() from on_replace() trigger is not
allowed.

Recursive invocation is not forbidden, but limited to the maximal
depth of 4 to avoid stack overflow.

Proudly fixes #587