Add option sync_on_start

[LeonidVas]

In fact, it is sometimes useful to synchronize all repositories at startup:

• you have some set of repositories without meta-information (with old metainformation)
• you have several small repositories running under the RWS, and it is good to update the metainformation on startup But sometimes it is not so good:
• you have more than 100500 repositories running under the RWS (it takes a very long time to start up)
• your service restarts often (the service goes to sleep if you use free dynos on heroku)

So, it is good to be able to control this functionality.