tether 0.1.0b3

Experimental

• neon: database is out of the object identity, as role already was. A
  Neon branch at an LSN is a snapshot of the whole project, so two objects on
  two databases of one project are one snapshot to tether: one pin branch per
  commit instead of two identical ones cut off the same LSN, one working
  branch per bookmark (as branch_scope already arranged), and open on
  each connects to its own database through them. No migration: existing
  pins keep their recorded ids and refs, stay referenced by the history that
  made them, and verify and repair as before; the next commit of content
  already pinned under an old id pins it once more under the new one.

Fixed

• commit pins once per pin id. Two objects with one identity and one
  content state name one snapshot; the second is now recorded at the state
  the pin was cut at instead of its own fingerprint of the same content,
  whose volatile address (a Neon LSN) could differ and made the backend
  refuse the second pin as hanging off the wrong LSN.