Splitstore Harness Test

[vyzo]

During the development of Splitstore v1 in #6474, we laregely relied on manual testing using live nodes in the mainnet.
While this is acceptable for development, and indeed very beneficial and confidence building to have the nodes running in mainnet, it is not sufficient for testing of production code.

We propose to create a harness test for splitstore, which includes a sufficient subset of the chain (at least 6 finalities) so as to initialize a splitstore node and trigger compaction, possibly starting from a snapshot.
We can't easily do that with an integration test, as we need a lot of (real world!) blocks and a sufficiently long chain to trigger at least one compaction.
We also need a sufficiently complex part of lotus so as to validate blocks and compute state during the test to ensure that everything is nominal.

This is a non-trivial project, but it is necessary in order to mark the splitstore production ready.