Add global sessionlimit to use for backends that have none configured

[fancycode]

Hello,

My understanding until now was the following: if a backend defines it, it gets its value of sessionlimit. If not it gets the value of a global sessionlimit, as defined in section [backend].

However this seems not the case, that is: there is no fallback sessionlimit (like there is one for secret). If so, when we don't specify a sessionlimit in the backend, there is no limit, correct?

Originally posted by @pmarini-nc in #792 (comment)