error during shutdown of a busy caching server (specifically, one
making many outgoing TCP connections).
Bug 1: resquery_destroy() accessed query->fctx->res->mctx, even though
query->fctx could already be destroyed at this point.
Bug 2: resquery_connected() called fctx_done(query->fctx, result)
immediately after fctx_cancelquery(), which could destroy *query.
Also added some explanatory comments.