Simply Relationship.next_free_id by depending on number of cached instances.

Only drawback: Setting @next_freed_id_counter to 1000 in tc_helper.rb is no longer possible. But this was useful mainly while adding / fixing test cases when implementing the Relationship instance cache.
Implement Relationship.clear_cached_instances, use it before serializing the package.

This is necessary to make serialization idempotent (i.e. make sure that Relationship instances are generated with the same IDs everytime the package is serialized). It also fixes a memory leak if Axlsx is used in a long running server process (eg a Rails app).