gimlet-seq-server: Allow A0 -> A0 and A2 -> A2 transitions

[jgallagher]

Currently, gimlet-seq-server's set_state_internal only allows A0 -> A2 and A2 -> A0 transitions; any other request results in Err(SeqError::IllegalTransition). This has ramifications for wicket doing updates: it wants to send sleds to A2, but if the sled is already in A2, that request fails. Wicket could ask the current state of the sled before sending the command to go to A2, but that introduces a (small) race window similar to a TOCTOU error. If we instead change gimlet-seq-server to allow A0 -> A0 and A2 -> A2 (which would do nothing except return Ok), that would allow wicket to always request the sled move to A2 regardless of its current state.

Would fix oxidecomputer/omicron#3644.

[jclulow]

Would it be possible to return a boolean rather than just unit, which said whether anything had to change to meet the request? Then you could at least decide whether you cared about that condition or not -- even if only for display purposes.

[jgallagher]

Changing the hubris interface to return a boolean is easy, yeah. Changing the MGS interface is trickier because it requires some version bumps to introduce a new response (and probably a new request too? need to think about that).

[labbott]

The hubris side of this was partially fixed by #2064 and fixes the mgs side by #2065

[hawkw]

Oh, I hadn't noticed this issue. I'll update #2065 to close this when it merges.