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`.
We can only hit this code if `$name` is a string, at which point we are only checking if it's already been used or not. Null name values are handled differently as before.