Limit the maximum number of tokens that can be locked

[dusan-maksimovic]

• Store max number of tokens that can be locked in SC config
• Introduce a new field to keep track of total tokens currently locked in SC (updated on each lock/unlock)
• Each time a user wants to lock some tokens, we check if the cap has been reached
• Add new MsgExecute option to allow the value in config to be updated by the whitelist admins
• Added a query that returns the number of total tokens currently locked
• Added test

[p-offtermatt]

LGTM!

I feel like we probably will need to document the endpoints a bit more at some point, but I think for the first version, this is ok. Maybe something naturally comes up when coordinating with the work on the frontend