Renames Sec{Read|Write}StateLimits

The Sec{Read|Write}StateLimits are filters related to the connections, when the
server did not know yet which vhost the request goes to. This means that once
those configrations are set it will be applied to entire server. This patch
just renames the Sec{Read|Write}StateLimits to SecConn{Read|Write}StateLimits
to make it more clear to the user. SecConnectionEngine was also renamed to
SecConnEngine.