timely-v0.32.0

This release changes how messages are stamped, and it removes the notificator and the operators that read a time off a capability.
Idle workers poll before they park, and progress tracking does less work on hot paths.

Breaking changes

• Message stamps are sets of times. A message's stamp is now a multiset of timestamps instead of one time.
  Singleton stamps behave as before.
  An empty stamp makes no progress claims. (#813)
• InputCapability::time() is now least(). It exists only for TotalOrder and returns Option<&T>.
  It returns None when no capabilities stamped the message.
  retain(port) is now retain_least(port), and the Deref to T is gone.
  Partially ordered times should use stamp() and retain_stamp(). (#817)
• The notificator is removed. Notificator, FrontierNotificator, unary_notify, and binary_notify are gone.
  To wait for a time, hold capabilities in a map keyed by time, and check them against the input frontiers. (#819)
• Operators that read a time off the capability are removed. This covers delay, delay_batch, delay_total, count, accumulate, aggregate, state_machine, branch, branch_when, and reclock.
  It also covers the flow-controlled iterator_source and ResultStream.
  For the container-generic form, use ok_err in core. (#818)
• Stamp deserialization requires T: Ord. Deserialization sorts the elements, because the bytes need not come from a valid Stamp.
  The bound applies to Stamp, Event, Message's Bytesable impl, and EventReader's EventIterator impl. (#823)
• SharedProgress has a new field, pushed. Code that builds it with a struct literal must set the field. (#832)

Added

• CapabilitySet can serve as a session capability.
  InputCapability::stamp() and retain_stamp() expose the full stamp. (#813)
• timely::scheduling::current reports the identifier of the operator scheduled on this thread.
  A host can call current::enter to charge outside work to an operator. (#831)
• WorkerConfig::idle_spin, set with --idle-spin MICROS, sets how long an idle worker polls before it parks.
  The default is 10 µs. (#816)

Fixed

• Counting pushers track a dirty bit explicitly.
  This fixes a missed flush reported in #824. (#825)
• Feedback edges drop messages with empty stamps.
  Those messages could circulate a loop without progress tracking seeing them. (#823)

Performance

• Workers poll before they park, signal peers less often, and serialize broadcasts once. (#816)
• Records that enter or leave a nested scope schedule their recipients in the same pass that moves them.
  This means fewer progress batches and less latency. (#832)
• Frontier updates skip a rebuild when a positive count cannot change the frontier.
  In one instrumented Differential BFS run, this avoided 9,808 of 71,806 rebuilds. (#829)
• Lazy progress reporting skips count scans it does not need. (#830)