Gantt perf: fixed that Project#start_date and #due_date run way too much queries.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11135 e93f8b46-1217-0410-a6f0-8f06a7374b81
Don't compute version start date if due_date is nil.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11136 e93f8b46-1217-0410-a6f0-8f06a7374b81
Check if index exists before removing it (#12713).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11137 e93f8b46-1217-0410-a6f0-8f06a7374b81