[calendar] deny fetch interval < 60000 and set 60000 in this case (prevent fetch loop failed)

[bugsounet]

Hi, I had the case of some users who set a very small fetchinterval (10 sec for example) in some cases, it may be that the request did not have time to complete correctly and that the next one has already been sent (generally on start of MM²) I think that lock min fetchInterval to 60000 is a good idea