Leaks and double frees in unsafe code

[rlei]

Multiple occurrences of leaks and (possible) double frees caused by unmatched Box::into_raw and Box::from_raw calls have be identified in current implementation.

see 6e6793b for an (incomplete) list of leaks.

[rlei]

For example, the summary from
valgrind --leak-check=full target/debug/deps/nonblockinghashmap-69f481c86edb5ffd --nocapture test_hashmap_concurrent_rw_no_resize:
(with num_keys lowered to 10_000 in test)

==28153== LEAK SUMMARY:
==28153==    definitely lost: 4,000,000 bytes in 250,000 blocks
==28153==    indirectly lost: 7,320,000 bytes in 440,000 blocks
==28153==      possibly lost: 0 bytes in 0 blocks
==28153==    still reachable: 32 bytes in 1 blocks
==28153==         suppressed: 0 bytes in 0 blocks
==28153== Reachable blocks (those to which a pointer was found) are not shown.

[rlei]

Commit 650d798 just kicked off the rewriting of the underlying key/value storage.

[rlei]

And ed9d9bc