Allow to define a prefix for bakery cookie name

In certain cases it is necessary to add a prefix to cookie name. For example, a site can stay behind a caching frontend proxy which strips all cookies and thus makes it impossible to authenticate on slave sites. However, there can be a rule set in this proxy to not strip cookies that start with a certain prefix. This feature will add a bit more flexibility to current cookie name altering mechanism - both prefix and suffix will be available.