Avoid dumping unused symbols/strings into the CDS archive

Currently all entries in the SymbolTable/StringTable are dumped into the CDS archive. The problems are

(1) We end up writing many unused items, such as the mangled names of hidden classes.

(2) We have to scan the symbol table inside a safepoint, which had caused bugs before (JDK-8245264).

Since JDK-8250990, we already maintain all used Symbols in a growable array. We can dump the shared symbol table using this array, and avoid walking the SymbolTable. A similar solution can be done for the StringTable.

This proposal will reduce complexity in the CDS code, and reduce the size of the CDS archives.