Speed up the tinkerpop test

[javeme]

There are several reasons for affecting the tinkerpop test speed:

1. rocksdb clear-schema would delete index-label too many times due to need to scan prefix
2. clear schema even if the database is empty
3. clear variables even if it's empty, and it will init variables schema at the same time if not exist
4. init and clear basic schema when load non basic graph