Merge branch 'master' into t

* master: (31 commits)
  go/zodb/zeo: Make "no serial on load after object deleted" to be a known bug
  go/zodb/zeo: Cosmetics
  go/zodb/zeo: Load: Fix thinko when rejecting result
  go/zodb/zeo: Add support for invalidations
  go/zodb/zeo: Teach zlink to serve incoming notification and requests
  go/zodb/zeo: MsgPack support
  go/zodb/zeo: Accept both int|bool for pickled message flags
  go/zodb/zeo: Factor-out tuple handling into encoding
  go/zodb/zeo: Factor-out string|bytes handling into encoding
  go/zodb/zeo: proto: Use enc.<X> and enc.as<X> to encode/decode type X
  go/zodb/zeo: Introduce notion of encoding
  go/zodb/zeo: Factor-out code to prepare pkt from message into pktEncode
  go/zodb/zeo: Start moving things related to protocol and messages encoding into proto.go
  go/zodb/zeo: Shutdown zlink on first error
  go/zodb/zeo: Use zlink as term for link in between peers
  go/zodb/zeo: Don't avoid defer
  go/zodb/zeo: errgroup -> xsync.WorkGroup
  go/zodb/zeo: Initial tests
  go/zodb/zeo: Fix thinko in openByURL error context
  go/internal/xtesting: LoadDBHistory
  ...