save index build timestamp, similar to ASYNC_REBUILD_TIMESTAMP, or ASYNC_CREATED_DATE
("SYNC_INDEX_CREATED_DATE" is my proposed name for SYNC case.)
Check IndexUtil.java for related code.
The reason this can be useful is: We saw a case index state stuck in 'b' for quite some long time. And without a timestamp to indicate where it started, it's hard to tell if this is a legit running task or stuck...