Keep a pointer to the avalanche processor in the Chainstate.
Set the pointer just after g_avalanche is initialized, to maintain the current behavior.
Note that we set avalanche on the active chainstate. For now this is reasonable as the active chainstate is always the for the entire duration of the `bitcoind` process, and is defined just after `LoadChainstate` is called.
This will change later when the `loadtxoutset` is implemented and is able to change the active chainstate dynamically, so we will have to unset avalanche on the background IBD chain and set it on the new snapshot chain. See assumeutxo.md.
Depends on D16260