Chunk class refactoring encountered while writing unit tests.

Good to have Chunk refactoring encountered while writing unit tests

1. Promote REF_COUNT_OFFSET to at-least protected access level and use it in GemFireChunkJUnitTest - search by REF_COUNT_OFFSET in the comments.

2. Promote MAX_REF_COUNT to at-least protected access level and use it in GemFireChunkJUnitTest.retainShouldThrowExceptionAfterMaxNumberOfTimesRetained

3. Promote IS_SERIALIZED_BIT & IS_COMPRESSED_BIT to at-least protected level and use it in GemFireChunkJUnitTest - search by IS_SERIALIZED_BIT in the comments

4. Chunk.getAddressForReading - can delete to Chunk.getUnsafeAddress, as both the methods are similar and `getUnsafeAddress` has more assert checks.