Address deprecations of `getThreadID()` and `setThreadID()`

[alexander-yevsyukov]

The console output is this:

/Users/sanders/Projects/Spine/logging/backends/jul-backend/src/main/java/io/spine/logging/backend/jul/AbstractJulRecord.java:287: warning: [deprecation] getThreadID() in LogRecord has been deprecated
    copy.setThreadID(getThreadID());
                     ^
/Users/sanders/Projects/Spine/logging/backends/jul-backend/src/main/java/io/spine/logging/backend/jul/AbstractJulRecord.java:287: warning: [deprecation] setThreadID(int) in LogRecord has been deprecated
    copy.setThreadID(getThreadID());

[alexander-yevsyukov]

Addressed in #96.