Summary
-------

Currently the default class data sharing (CDS) setting for Server VM is `-Xshare:off`. Change
the default to `-Xshare:auto` to make it easier to use CDS.

Problem
-------

We expect CDS to be widely used with the Server VM. However, the
current default CDS setting for Server VM is `-Xshare:off`. This makes it
cumbersome to use CDS.

Solution
--------

Change the default to `-Xshare:auto`, so as long as a CDS archive exists
in the JDK, CDS will be automatically used without specifying extra flags.

In JDK 8 and before, RewriteBytecodes was disabled when CDS was enabled.
This caused performance degradation in the Server Compiler (aka C2). However
this has been fixed in [JDK-8074345](https://bugs.openjdk.java.net/browse/JDK-8074345) since JDK 9, so there's no longer need
to disable CDS by default with the Server VM.

Specification
-------------

For the Server VM, if the `-Xshare` option is not explicitly specified in the command-line: 

 - If the `-XX:SharedArchiveFile=<file>` option is specified, and `<file>` exists, the JVM will attempt to map `<file>` as the CDS archive, as if `-Xshare:auto` had been specified.
 - If the `-XX:SharedArchiveFile` option is not specified, the VM will attempt to map the default CDS archive  ( `$JAVA_HOME/lib/server/classes.jsa`), as if `-Xshare:auto` had been specified.

Note: no user visible changes are needed in the `-help` output because the current output for the server VM already says (incorrectly)

```
    -Xshare:auto      use shared class data if possible (default)
    -Xshare:off       do not attempt to use shared class data
    -Xshare:on        require using shared class data, otherwise fail.
```
so in a way we are just changing the implementation to match the documentation.