[FIX] Return the last blocks when fetching the ommers ancestors from memory

[ntallar]

Description

Addresses issue on testnet

Scenario

• Importing a chain of 12 blocks (1858 - 1870)
• Block 1870 had an ommer of number 1865 (it's chain ancestor is then block 1864)

When importing block 1970:

1. Blocks 1858-1869 were returned from the BlockQueue when fetching the branch from it
2. Taking 6 blocks from it resulted in blocks: 1858-1863

The above chain didn't have the ommer ancestor, so the validation of them failed with OmmersAncestorsError. As the above mentioned chain was followed by 3/5 miners they forked irrecoverably.

Proposed Solution

Changing Taking 6 blocks from it resulted in blocks: 1858-1863 to Taking the last 6 blocks from it resulted in blocks: 1864-1869

Testing

Due to the complexity of testing this scenario and the time requirements for the having the internal testnet running no test was added.

[mirkoAlic]

LGTM! please create a task for future analysis over block queue in order to see how we could start adding tests for this kind of things.