ServerService used to own three mutexes coordinating six pieces of
mutable state (exports, busy, controlState, controlSeq, controlSubs,
LeaseManager). The lock-ordering rule that fixed the import/attach
race in 0c892411a lived only as a comment. Two-mutex (fast/slow)
ledger absorbs all of it; the two mutexes are never held
simultaneously, so the ordering rule becomes a property of the type
instead of a discipline callers must observe. TryReserveForImport
closes the window where two concurrent OpReqImports could both pass
the busy check before either marked busy.
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