Handle data loss from in-progress spans

[fractalwrench]

Goal

In-progress spans are currently lost when the process terminates. We should come up with a solution that addresses this pain point as process termination is much more common on mobile than in a backend environment.

One option would be to persist any update to a span by serializing protobuf to a memory-mapped file. If the process terminates, these snapshots can be deserialized and exported. Doing this would require ending the span and likely setting an attribute/information to indicate that the span was resurrected from a snapshot, rather than explicitly ended.