Introduce rusty_leveldb_sgx as persistence storage

[m4sterchain]

Description

As described in #103, I have ported the rusty_leveldb crate into sgx with test cases added in functional_test.

Fixes # (issue)

Type of change (select applied and DELETE the others)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just sync with upstream third-party crates

How Has This Been Tested?

https://ci.mesalock-linux.org/m4sterchain/incubator-mesatee/44

Checklist (check ALL before submitting PR, even not applicable)

• Fork the repo and create your branch from master.
• If you've added code that should be tested, add tests.
• If you've changed APIs, update the documentation.
• Ensure the tests pass (see CI results).
• Make sure your code lints/format.