Previously, JarFileArchive would always unpack any entries marked for
unpacking to ${java.io.tmpdir}/spring-boot-libs. This could cause
problems if multiple Spring Boot applications were running on the same
host:

- If the apps are run as different users the first application would
  create the spring-boot-libs directory and the second and subsequent
  applications may not have write permissions to that directory
- Multiple apps may overwrite each others unpacked libs. At best this
  will mean one copy of a jar is overwritten with another identical
  copy. At worst the jars may have different contents so that some of
  the contents of the original jar disappear unexpectedly.

This commit updates JarFileArchive to use an application-specific
location when unpacking libs. A directory beneath ${java.io.tmpdir} is
still used but it's now named <jar-file-name>-spring-boot-libs-<uuid>.
A loop, limited to 1000 attempts, is used to avoid problems caused by
the uuid clashing.

Closes gh-4124