Merged cache fixes from 3.8 branch:

- The cache used an excessive amount of memory, causing applications
  with large caches to exhaust available memory.

- Fixed a number of bugs in the handling of persistent ZEO caches:

- Cache records are written in several steps.  If a process exits
    after writing begins and before it is finishes, the cache will be
    corrupt on restart.  The way records are writted was changed to
    make cache record updates atomic.

- There was no lock file to prevent opening a cache multiple times
    at once, which would lead to corruption.  Persistent caches now
    use lock files, in the same way that file storages do.

- A bug in the cache-opening logic led to cache failure in the
    unlikely event that a cache has no free blocks.