Ticket #610 (task)

Try to do these late and in one go:

• Remove tombstoned flag - going for a different approach
• Add tables etc for webdav synch - details later
• Use utility classes to turn recurrence and exception rules into a concatenated string in the db. Loses 2 retrievals from collections
• Make calendar summary into displayname collection
• Make calendar description into collection
• Add parent-id column and create a unique index on name + parent-id. Avoids window in which duplicate calendars get created. (See below)

When adding a collection we check for uniqueness of the path. However, we don't have a unique index on the path due to length limitations. So 2 simultaneous transactions can go ahead and create the same collection. We need a parent id column which we can index with the name to ensure that we get a failure at commit.