[db] Share db for two contract indexer

[envestcc]

Description

We want to combine db files of two staking contract indexer, which helps to make easy to maintain.

I introduce the new KVStoreShards to make different shard(with different bucket prefix of a same kvstore) of the kvstore. Therefore, the two indexer can use different shard of same kvstore.

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• Code refactor or improvement

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [] make test
• [] fullsync
• [] Other test (please specify)

Test Configuration:

• Firmware version:
• Hardware:
• Toolchain:
• SDK:

Checklist:

• [] My code follows the style guidelines of this project
• [] I have performed a self-review of my code
• [] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [] My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• [] New and existing unit tests pass locally with my changes
• [] Any dependent changes have been merged and published in downstream modules