Explore using mutex/locks and transactions on certain queries

[jurajhrib]

Some queries use naive approach for reading/writing to DB. It might cause some race conditions when a larger number of online users interact with the app. Locking db and using transactions on more places might help to prevent this.

Mainly when:

• saving tip (and creating generation)
• creating run
• importing questions (this might be solved by temporarily pausing the game for everyone while importing)