[Blockchain/IBFT] Update snapshot after processBlock ends

[dbrajovic]

Description

This PR delays the call to processHeaders() until after the block has been verified and processed completely.

Previously, snapshot updating was done during verifyHeaders() despite processBlock() potentially failing afterwards.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Tested manually with a 4-node cluster. Steps:

1. Spin up 4 nodes (develop build)
2. Sequentially restart all nodes (fix/snapshot_update build)
3. Nothing breaks

Tested PoA voting mechanism (manually). Steps:

1. Spin up 4 nodes (fix/snapshot_update build)
2. While the cluster is running, spin up a 5th node (--seal)
3. Cluster votes for 5th node to be included in the validator set.
4. Observe the 5th node is able to join consensus on new blocks.

Additional comments

Fixes EDGE-465

[Kourin1996]

LGTM