Zend Db Query Builder Optimisation

Found that the query builder was spending a long time building queries with large arrays in the where clause. After a little bit of profiling noticed that line 111 was been called a lot. `array_search` is an expensive lookup, especially as the return value wasn't been used beyond checking it wasn't false, so replaced with a less expensive `isset`.
`$position` is only checked to see if it is explicity false (see line 117), which `isset` will return if it's not set.