Allow UPDATE by expression

[freshp86]

Currently UPDATE queries can only accept literal values. For example

db.update(employee)
  .set(employee.salary, 2000)
  .where(employee.hireDate.lt(...));

There is no way to calculate the new value based on existing values. Proposing
the following syntax to allow update-by-expression.

/**
 * typedef {{
 *   vars: !Array<!lf.schema.BaseColumn>
 *   fn: !Function
 * }}
 */
lf.query.UpdateExpression;

• Example1: updating some employees salary by 10%.

var updateExpression = {
  vars: [employee.salary],
  fn: function(x) { return x * 1.1; }
};
db.update(employee).
  set(employee.salary, updateExpression).
  where(employee.hireDate.lt(....));

• Example2: Combining values from multiple colmuns. Converting the bonus to salary.

var updateExpression = {
  vars: [employee.salary, employee.bonus],
  fn: function(a, b) { return a + b; }
};
db.update(employee).
  set(employee.salary, updateExpression).
  set(employee.bonus, 0).
  where(employee.hireDate.lt(....));