Remove magic number usage for last assigned partition id

[yittg]

While reading code about partition spec, i find some usages of magic number 999 which should be PARTITION_DATA_ID_START - 1 i think. Fix me if i misunderstand.

@rdblue would you help to review if you have time?

[rdblue]

I don't think this should be public.

[yittg]

change it to package level.

[rdblue]

Looks good overall. Thanks, @yittg!

[yittg]

@rdblue would you like to merge this one?

[yittg]

It is private now, @rdblue

[stevenzwu]

There is a test failure that seems unrelated to this PR. cc @kbendick

org.apache.iceberg.flink.TestFlinkUpsert > testPrimaryKeyFieldsAtBeginningOfSchema[catalogName=testhadoop, baseNamespace=default, format=PARQUET, isStreaming=true] FAILED
    java.lang.AssertionError: 
    Expecting:
      [+I[bbb, 2022-03-01, 3], +I[aaa, 2022-03-01, 1]]
    to contain exactly in any order:
      [+I[aaa, 2022-03-01, 2], +I[bbb, 2022-03-01, 3]]
    elements not found:
      [+I[aaa, 2022-03-01, 2]]
    and elements not expected:
      [+I[aaa, 2022-03-01, 1]]
        at org.apache.iceberg.flink.TestHelpers.assertRows(TestHelpers.java:137)
        at org.apache.iceberg.flink.TestFlinkUpsert.testPrimaryKeyFieldsAtBeginningOfSchema(TestFlinkUpsert.java:[219](https://github.com/apache/iceberg/runs/5834902185?check_suite_focus=true#step:6:219))

[yittg]

Thanks @stevenzwu .

And shall we open an issue to track this fragile test case, @kbendick .
#4515 is exactly this issue