Use a single lock file for stages and a single file for prefixes.

- Locks now use fcntl range locks on a single file.

How it works for prefixes:

- Each lock is a byte range lock on the nth byte of a file.

- The lock file is ``spack.installed_db.prefix_lock`` -- the DB tells us
  what to call it and it lives alongside the install DB.  n is the
  sys.maxsize-bit prefix of the DAG hash.

For stages, we take the sha1 of the stage name and use that to select a
byte to lock.

With 100 concurrent builds, the likelihood of a false lock collision is
~5.36e-16, so this scheme should retain more than sufficient paralellism
(with no chance of false negatives), and get us reader-writer lock
semantics with a single file, so no need to clean up lots of lock files.

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