Bugfix in rehash (probably last bug).

Calls to ck_hash_item() from ck_rehash() for items in hash table
  in some cases used the same pointer for to_hash and free
  parameters.
Removed function ck_insert_to_buffer() (functionality moved to
  ck_insert_item).
Some other minor changes.
TODO: refactor ck_rehash() (too long) + replace table1 and table2
      by array of tables (will be helpful also for d-ary hashing).