OPENDJ-1719: implement upgrade tasks for migrating JE backends

The task comprises of several steps:

* aborts the upgrade if there are enabled local-db backends but no JE
  in the class-path
* aborts the upgrade if the enabled local-db backends cannot be opened
  or if the upgrade task does not support automated migration, in which
  case the user must manually export their data, disable the backend(s),
  upgrade, and re-import
* migrates any local-db backend configuration to the new je-backend
  config model
* prepares existing local-db backend databases for a rebuild-all by
  making the database compatible with the pluggable storge architecture.
  In particular, the task renames the compressed schema indexes and
  id2entry to the naming scheme used in the new JE backend, and removes
  all other indexes. The updates are performed as a transaction to
  prevent partial migration
* registers the rebuild-all post-upgrade task.