Rename WikiPage::isParserCacheUsed to WikiPage::shouldCheckParserCache

Rename WikiPage::isParserCacheUsed to WikiPage::shouldCheckParserCache

'isParserCachedUsed' implies that the parser cache usage has already occurred,
and obscures the true purpose of this method, which is to determine whether or
not the requested page *should* be looked up in the parser cache.

Only usage in extensions is in TextExtracts, which I changed to be both
backward- and forward-compatible in If5d5da8eab13.

Change-Id: I7de67937f0e57b1dffb466319192e4d400b867de