Don't store `transaction` on `trace` object, as the latter might be shared amongst threads
Don't store `transaction` on `trace` object, as the latter might be shared amongst threads

Fixes #144

beniwohli/apm-agent-python,beniwohli/apm-agent-python,patrys/opbeat_python,beniwohli/apm-agent-python,patrys/opbeat_python,patrys/opbeat_python,beniwohli/apm-agent-python,patrys/opbeat_python