New sequence ID in messages cannot work

The previous logging enhancements adds Context.sequence_ids, seemingly under the assumption that every timed activity message will get a unique and new sequence_id.

This is entirely broken, as it makes the assumption that BuildStream uses threads.

Child tasks are run in child processes with copy-on-write memory, as such there is no way for a plugin to corrupt main memory or ever cause a lockup; it also means that any data communicated with a child task must be done over the IPC, memory cannot be shared.

Marking this as critical, this needs to be fixed or removed before the misleading code causes other contributors to get confused.