WL#2499: Use same savepoint id for an entire SQL statement

This WL item is connected to WL 2443.
For WL 2443 to work it is necessary to perform all reads in an UPDATE
statement using the savepoint id at start of the statement. If not then
we are going to see our own updates and perform additional updates based
on this information.

Currently this works since the MySQL Server performs a pre-read in the
difficult cases. This means that we perform 2 reads before updating
and we would like to go down to 1 read at most and in many cases even
down to zero reads before updating.

Should be possible to solve entirely in the NDB handler