Add read-only KVStore fsTimings to timing stats

During investigation of slow background fetches on a customer environment, I found that while we record filesystem timings for both read-only and read-write KVStores, we only report the timings for read-write:

void KVStore::addTimingStats(ADD_STAT add_stat, const void *c) {

    /* We don't support any timing stats for read-only instances

     * at this point.

     */

    if (isReadOnly()) {

        return;

    }

To better assist in analysing read timings, we should also include the read-only instance in the stats (accessed via the kvtimings cbstats group).