MGenServer: Implement maintenance mode

[rualark]

There can actually be two maintenance modes:

• New tasks cannot be started except for admin. This mode is often needed when some changes or experiments are made in Reaper - because without it user tasks will need to be stopped, which is not good. As entering this mode often requires to stop user's tasks, it is good if there is a banner at the top of page. This happens often when there is a need to debug something in reaper.
• No changes can be made, which actually means that logged in user except admin should not see anything. This is very rarely needed - only when significant changes to database or files are made. This happens about 2 times per year.

[rualark]

Another approach to "no processing" mode is to keep tasks in queue without preventing user from starting tasks. This is a better idea because user's workflow does not change except for need to wait longer.

If user's task is stopped, it should be also put in queue. To do this:

1. Set maintenance parameter in server.pl
2. Restart server (this will automatically queue running task.
3. Do maintenance
4. Reset maintenance parameter in server.pl
5. Restart server

[rualark]

If there are multiple servers, each server can be in maintenance mode separately. This means that some server will run only admin tasks, while other servers will run any tasks.