RFR 8251557 Avoid dumping unused symbols/strings into the CDS archive

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8251557
http://cr.openjdk.java.net/~iklam/jdk16/8251557-cds-dont-dump-unused-symbols.v01/

Part of CDS code clean up:

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.

For StringTable, we scan all archived classes, and dump only the
interned strings used by those classes (plus extra strings
specified by SharedArchiveConfigFile) into the CDS interned string
table.

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

Testing - tiers 1-4

Archive file size for "java -Xshare:dump"

    old: 39,368 symbols 7,028 strings 11,784,200 bytes
    new: 37,387 symbols 7,020 strings 11,683,320 bytes (~0.9% reduction)

Thanks
- Ioi