Ssl::ServerPeeker must become a store_client to prevent [error] entry trimming

Ssl::ServerPeeker must become a store_client to prevent [error] entry trimming

We do not need a store client during the certificate peeking stage because we
do not send the error to the client and only accumulate what is being written
to the store. However, if there is no store client then Store will trim the
entry and we will hit a

  store_client.cc:198: "entry->swap_filen > -1 || entry->swappingOut()"

assertion when we finally try to use it to serve the error response.