RFR(XS) 8231278 Rename FileMapHeader::_read_only_tables_start to _serialized_data_start

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8231278
http://cr.openjdk.java.net/~iklam/jdk14/8231278-rename-read_only_tables_start.v01/

The name _read_only_tables_start is ambiguous, as there are multiple 
read-only
tables in the CDS archive. It should be renamed to _serialized_data_start,
as that where the data is accessed by {ReadClosure,WriteClosure}::serialize.

Thanks
- Ioi