v1.0.0-rc.2

Changes:

• CubDB.put_and_delete_multi/3 to atomically put and delete entries
• CubDB.put_new/3 to put an entry only if the key does not exist yet
• More efficient implementation of CubDB.put_multi/2 and
  CubDB.delete_multi/2
• CubDB.get_multi/2 does not block writers