Improve DB requests.

[pxp9]

• maybe you should preload (with left join) default city when fetching a task so you don't have to make one more request when fetching the default city

• several write operations in a single command should be executed inside of a transaction so db can be always consistent
  this comment is for all all commands but can be addressed in a single pr after this one is merged. because will require a lot of work