fix(p2p): slashing test + slasher client does not store own state

[Maddiaa0]

Overview

The problem

• the function being used to progress the chain had recently been upgraded to use the redeployed accounts, but this test reorgs them out so they were failing
• Secondly and most importantly, there was a race condition when detecting the prune.
  • In order to workout the deadline for the vote, it must work out the current slot, to do so, it was getting it from the block header, which was actively being reorged out. This has been replaced with a reference to the epoch cache

Adds methods to the archiver such that the slasher_client does not need to maintain its own tips. I believe that i can make these changes to the p2p client as well

[Maddiaa0]

• feat(p2p): no tips store in the p2p client #12395
• fix(p2p): slashing test + slasher client does not store own state #12380 👈 (View in Graphite)
• master

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[LHerskind]

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