Implement multiple-table UPDATE feature.

CS as of 1.4.2 doesn't support multiple-table UPDATE feature.
Given that now CS has a number of requirements imposed by JOIN-s we can only support multiple-table UPDATE-s that satisfy this requirements.
Here are the current limitations:

• CS can execute only equi-JOIN operations
• It can't have a circular graph of JOIN keys(the workaround is to use functional JOIN keys)
  Consider the example of multiple-tables UPDATE-s CS could support:

  UPDATE tab1, tab2 SET tab1.column1 = value1, tab1.column2 = value2 WHERE tab1.id = tab2.id;