Make the shared Key.SDF date formatter thread-safe (CodeQL java/thread-unsafe-dateformat)

[vharseko]

Summary

Resolves the java/thread-unsafe-dateformat CodeQL alert on Key.SDF.

Key.SDF was a public final static SimpleDateFormat. SimpleDateFormat
is not thread-safe — concurrent format() / parse() calls mutate its
internal Calendar and can produce garbled text, wrong dates, or
ArrayIndexOutOfBoundsException. This single shared instance is reachable
from several classes that run on multiple threads:

• Key.decodeDisplayable / Key.toString
• Value.toString
• KeyParser (parse, date keys)
• com.persistit.ui.ValueInspectorTreeNode (Swing)

Fix

Wrap the formatter in a ThreadLocal so each thread gets its own instance:

public final static ThreadLocal<SimpleDateFormat> SDF = ThreadLocal
        .withInitial(() -> new SimpleDateFormat("yyyyMMddHHmmss.SSSZ"));

and update the four call sites to Key.SDF.get().format(...) /
Key.SDF.get().parse(...).

The date pattern is unchanged, so formatting and parsing behaviour is
identical for every input; only the shared mutable state is removed.

Note

Key.SDF is a public field, so its compile-time type changes from
SimpleDateFormat to ThreadLocal<SimpleDateFormat>. All in-repo callers
are updated in this PR. Any external caller would replace Key.SDF.format(d)
with Key.SDF.get().format(d).

The other, private static SimpleDateFormat fields in the tree
(JournalTool, VolumeHeader, CheckpointManager, and the AbstractSuite
test) were not flagged by CodeQL and are left unchanged.

Testing

mvn -o -pl persistit/core,persistit/ui -am compile — BUILD SUCCESS.