Rename some api endpoints and keys
get-tree-head-to-cosign --> get-next-tree-head, get-tree-head-cosigned --> get-tree-head, tree_size --> size, inclusion_path --> node_hash, consistency_path --> node_hash
Proposed in https://git.glasklar.is/sigsum/project/documentation/-/blob/main/proposals/2022-07-api-renaming.md.