refactor: StateManagerImpl::take_tip

[mraszyk]

This PR refactors StateManagerImpl::take_tip:

• it inlines the case tip_height > StateManagerImpl::INITIAL_STATE_HEIGHT of the closure hash_at into the case snapshot.height > tip_height of a state obtained via state sync;
• it replaces the ad-hoc implementation of the case tip_height == StateManagerImpl::INITIAL_STATE_HEIGHT by using StateManagerImpl::compute_certification_metadata so that the new implementation could be used also for tip heights other than StateManagerImpl::INITIAL_STATE_HEIGHT.

This PR also add a new metric state_manager_tip_hash_count tracking the number of tip heights whose state snapshot was not stored by this node and whose state hash was computed by this node. Currently, the only such state is the initial state and thus the metric is expected to be 1 in tests (starting from the initial state) and 0 in production (resuming from a checkpoint).