Fix state inconsistency in Orleans Chirper account

[TimonPasslick]

Summary

Disabled reentrancy/interleaving for FollowUserIdAsync‎ and UnfollowUserIdAsync. Interleaving can cause 2 problems if both methods get called with the same username on the same grain:

1. If a user gets followed, but between the remote and local update, he gets unfollowed, the local state says the user is followed, but the remote state says the user is not followed. Same vice versa.
2. The follow and unfollow commands may arrive on the other grain in reverse of sending order, causing the same inconsistencies.

Fixes #7124

[TimonPasslick]

@dotnet-policy-service agree