[HDDS-4651] Implement Distributed Sequence ID Generator - ASF JIRA

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SCM allocates ids in a batch way. It maintains three fields: firstId, lastId and nextId. It saves the lastId in rocksDB, saves firstId and lastId in memory. The initial value of nextId is firstId.
When getNextId() is called, if nextId is less than lastId, SCM just returns nextId and increases it, otherwise it calls allocateBatch(expectedLastId, newLastId) to allocate a batch of ids, then serves the request.
In allocateBatch(expectedLastId, newLastId) , it works in CAS way:
it reads lastId from rocksDB.
if lastId equals expectedLastId, it saves newLastId into rocksDB, returns success, otherwise it rejects the allocation request.
It also provides a getLastId() to read lastId from rocksDB. The allocation works in a loop way:

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