Pre-keys are refreshed at regular (throttled) intervals, but the
"refresh" operation will only actually upload new pre-keys if the
server says we're running low. In addition, after the "refresh",
there's a "rotation" operation for the signed pre-key if the current
one is more than two days old. This only runs if the refresh was
skipped, since uploading new one-time pre-keys is already an
opportunity to rotate the signed pre-key (and it's worth doing so,
since pre-keys running low can indicate high traffic).
None of this has changed. However, we can now do this for both ACI and
PNI pre-keys. This reworked +[TSPreKeyManager
checkPreKeysWithShouldThrottle:] quite a bit to declare the proper
operation dependencies, which isn't *strictly* necessary because the
queue they're executed on is already limited to a
maxConcurrentOperationCount of 1. We could remove that at this point
with proper use of "barrier" operations, but since all the operations
are hitting the server and account updates have to be serialized on
*that* end, there doesn't seem to be much of a benefit.
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