Fix memory leak when dropping a sync or future cache.

[tatsuya6502]

Fixes #176

Problem and Cause

When a sync or future cache was dropped, value part of cache entries were left allocated (leaked). This was due to a bug in the Drop::drop method of moka::cht::SegmentedHashMap; it was using bucket::defer_acquire_destroy function to drop any entry (a bucket struct), but that function only drops the key part of the entry.

Fix

Fixed the bug by updating the drop method of moka::cht::SegmentedCache to use the followings:

• For a live bucket (having both the key and value), use bucket::defer_destroy_bucket function to drop both the key and value.
• For a dead bucket (having the tombstone bit set and having only the key), continue using bucket::defer_acquire_destroy function to drop only the key.

Verification

Verified the fix by the following methods:

• Ran the reproducing code using valgrind and confirmed that the memory leak was fixed.
  • "definitely lost: 0 bytes in 0 blocks"
  • "indirectly lost: 0 bytes in 0 blocks"
• Added a unit test moka::cht::segment::test::drop_map_after_concurrent_updates to ensure that the drop method of both the key and value types are called when the map is dropped.

[tatsuya6502]

Merging.

[tatsuya6502]

Note: I have updated mokabench, the load generator we use for pre-release testing, to track heap allocations (commit: moka-rs/mokabench@bec3976). I ran it against Moka v0.9.3 (without the fix in this PR) and v0.9.4 (with the fix). I confirmed that v0.9.4 has nothing to indicate any other memory leaks.