Deprecate a few one-line global functions

Additions to the PHP language made these redundant.

* wfArrayLookup() is similar to PHP 5.1's array_intersect_key()
  yet has serious limitations. For example, integer string values
  are implicitly cast to integers.

* wfArrayMerge() can be replaced by PHP 5.3's array_replace().

* wfTime() just returns microtime( true ).

Change-Id: I2c6844fc48a265d2d885083b5bed8df846e0aaf4