Only update the cache on cache miss

Change I9b1d68c4faa8d177d8d0088fa1a5879caed4f1fe added an indiscriminant call
to writeCache(). I assume that it was supposed to be regulated by the check for
$this->isRecall() (which is a getter for '$this->recall'), but there was no
concomitant change to ensure that recall is set true on cache hits.

My fix does not set recall to true. The style of "pass-by-attribute" is tricky
and should be factored out by a subsequent change. This change moves the call
to writeCache() so that it is only called on cache miss.

Change-Id: I9b690e55001859c97fd40330272791d49ec6de75