Sql. Avoid spamming execution tasks when possible

Need to revise all usages of org.apache.ignite.internal.sql.engine.exec.ExecutionContext#execute and check whether spawning a new task is legit in every particular case or it's better to do the work right now.

For example, lets take a look at org.apache.ignite.internal.sql.engine.exec.rel.ScanNode#request:

@Override
    public void request(int rowsCnt) throws Exception {
        assert rowsCnt > 0 && requested == 0 : "rowsCnt=" + rowsCnt + ", requested=" + requested;

        checkState();

        requested = rowsCnt;

        if (!inLoop) {
            context().execute(this::push, this::onError);
        }
    }

in case of the very first request we will spawn a new task, but it would be better to drain the first batch of rows as well.