Fix mem leaks in gnome-shell-calendar-server

I've been seeing my gnome-shell-calendar-server grow in mem usage by multiple GBs in a day or two for a little while. I finally tracked down a few possible causes with valgrind, patched and tested a version of ubuntu's 3.36.3 gnome-shell package with these changes, and haven't seen memory usage grow in a day of leaving it running while doing calendar-related & other tasks.