[avalanche] initialize g_avalanche after loading the block chain

Summary:
Initializing Avalanche requires the ChainstateManager to be initialized first. In a next commit, when backporting https://github.com/bitcoin/bitcoin/pull/22564/commits/5921b863e39e5c3997895ffee1c87159e37a5d6f, the chainman will be initialized after the loading of the block chain.

As far as I can tell this is OK because at this point the Avalanche event loop is not yet started, and the other components don't use g_avalanche or nLocalServices at this point. These objects are both used when the node is started at step 12.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13081