dispatch: rebalance users with a fixed count

[andydunstall]

Fixes #2091

• Resets the user worker count before rebalancing since _user_gen compares the expected number of fixed count users with the users in self._users_on_workers
• Sets self._try_dispatch_fixed = True to include fixed users in the rebalance
• Extends TestAddWorker and TestRemoveWorker to add a test for fixed count (which both fail without this commit) - these just copy test_remove_worker_during_ramp_up and test_add_worker_during_ramp_up with a fixed user

[EzR1d3r]

Looks fine. @cyberw @mboutet can you review too?

[cyberw]

Lgtm, but @mboutet is the user distribution expert :)

[mboutet]

LGTM also! Thank you all.