Audit buffer / block-creation lock ordering for deadlock risk

Follow-up from #13 review: in `src/rest/routes.rs`, the buffer lock could deadlock the block-creation lock if it isn't guaranteed to be released. Audit every site that acquires either lock to confirm the ordering is safe.

Review comment: https://github.com/ProvableHQ/aleo-devnode/pull/13#discussion_r3314856262