STM: refine the notion of "simply a tactic"

When a worker sends back a system state to master, it tries to
just send a delta.  If the command is a simple tactic, then only
the proof state changes.

Now, what is a simple tactic?
1. a tactic
2. that does not change the environment

Check 1. was surely incomplete.  Now it is:
  VernacSolve _ | VernacFocus _ | VernacUnfocus | VernacBullet _
Is it complete?

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