Update the search index while moving pages, assume the index is correct

This simplifies the search for links because now it can be assumed that
the search index is always correct and contains even link updates that
haven't been executed yet. The problem before this change was that the
search index became unusable after a namespace move.
Reconstruct missing id in IO_WIKIPAGE_READ and PARSER_CACHE_USE events

When p_cached_instructions() is called without the $id parameter the id
is also missing in the IO_WIKIPAGE_READ and PARSER_CACHE_USE events.
However in both events the file path is supplied and using pathID() the
path can be used for getting the id.