[v0.7.4-beta] Fix race condition

[I-Iugo]

Sequel already handles primary key incrementation, in addtion this line is not threadsafe. With this line, it's possible to create several users with identical ids at the same time.

Words from @jeremyevans :arrow_down:

The users table still seems to use a single primary key, not a composite primary key. I would delete the entire line as it is dangerous. If you run anything more than a single thread, single process application, it has a race condition that will cause it to fail.

[ccammilleri]

PR accepted, thanks @HugoPouliquen