feat: mpool: Cache actors in lite mode

[magik6k]

Related Issues

Mpool startup in lite mode for some reason requests the same actor head 1000s of times (probably for each message ever sent)

Proposed Changes

Additional Info

This makes my node start in ~2 seconds instead of roughly 20min when backed by a lite node

Likely there is a better fix to the underlying issue like cleaning up messages in mpool after finality, but this is way simpler and also solves the issue.

Checklist

Before you mark the PR ready for review, please make sure that:

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• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
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• New features have usage guidelines and / or documentation updates in
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• Tests exist for new functionality or change in behavior
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