RFR(S) 8210875 Refactor CompactHashtable

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8210875
http://cr.openjdk.java.net/~iklam/jdk12/refactor-compact-hash.v01/

The template class CompactHashtable contains code for symbols
and strings, which should really be provided by the instantiations
of this template (i.e., {StringTable,SymbolTable}::_shared_table).

I have refactored CompactHashtable 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.)

Thanks
- Ioi