SEC seems to stop handling requests while inside SecStore

[davepacheco]

While working on oxidecomputer/omicron#6215, I ran into a bug (in my code) that caused the Nexus impl of SecStore to hang inside saga_update. While that was going on, requests to Nexus to list sagas hung. That was surprising. SecStore functions can be talking to storage over the network and the SEC should not become unresponsive while that's happening.

[davepacheco]

I should clarify: what I observed was that Nexus's internal saga list API hung. It's conceivable that it was hanging somewhere else (e.g., trying to talk to the database).