0.18.8

Two runs writing one manifest no longer lose an entry.

Fixed

• Two runs writing one manifest no longer lose an entry. The merge added
  for this in 0.4 re-read after writing and retried if its own keys were gone,
  but a writer only checked its own keys: one that read before another's write
  could rename over it afterwards, and both reported success. The
  parallel-process test caught it under load in the 0.18.7 release run. The
  read, merge and write now happen under a lock file; a lock whose process has
  died, or older than any write takes, is taken over, so a killed run cannot
  wedge an install.