Backported following changes from vNext into the v1.6:

• Replace raw disk files (i.e. blocks and TXs) with the DB storage.
• Prevent TX submission if nonce is not greater than current address nonce.
• Delete TXs below min fee from the pool.
• Provide TX pool info by address.

• Added support for installing the node on CentOS 8.

• Fixed the issue with inconsistent block configuration structure at the activation of dormant validator logic.

• Intermediate release for diagnostic purpose

• Activate dormant validator logic on MAINNET at block 1330000.

• Activated last proposed block tracking on MAINNET at block 1300000.

• Removed tracking of last proposed block until defined point in chain, to ensure full sync compatibility.

• Activated dormant validator handling on TESTNET.

• Implemented automatic disabling of dormant validators.

• Don't allow removing a validator at the point of inclusion in the new active set.
• Exposed additional fields in validators API endpoint.