Refactor CompactHashtable

The template class CompactHashtable contains code for symbols and strings, which should really be provided by the instantiation of this template (e.g., SymbolTable::_shared_table).

See the problem in http://hg.openjdk.java.net/jdk/jdk/file/9c6d5e31618e/src/hotspot/share/classfile/compactHashtable.inline.hpp

CompactHashtable should be refactored to make it similar to ResourceHashtable. That way we can more easily create other instantiations, which would be handy in JDK-8210388 (Use hash table to store archived subgraph_info records.)