BasicHashtble's destructor only frees hashtable buckets, but not entries in the hashtable, that results a lot of duplicated code of its subclasses to cleanup hashtable entries.

The duplicated code can be consolidated into the base class.

There is a discussion here: https://github.com/openjdk/jdk/pull/8148#issuecomment-1092088844

Also, there is no point to complicate Entry allocation: first allocate memory, then invoke placement new.