Support for escape char in "like" expressions

[andrus]

There should be a way to redefine an "escape" char in "like" expressions, so that users could build queries with '%' and '_' not treated as wildcards. E.g.:

name like 'A$%bcd%' escape '$'

// matches "A%bcde", but not "Aabcde"

Now that #582 is implemented, we are no longer relying on Cayenne expression parser (which does not support escape chars). So we can implement this now.