Retain segment in memory on QuotaExceededError

[joeyparrish]

When MediaSource throws QuotaExceededError, we reduce the buffering goal to try to avoid subsequent errors. But when MediaSource rejects this media segment, we don't retain it in memory. The pipeline is entirely ephemeral, and we end up downloading that segment again.

Ideally, we could retain the segment in StreamingEngine in memory until it has been accepted by MediaSourceEngine. This will depend on the same architecture changes needed for #880, in which StreamingEngine will need to be generally capable of retaining segments without MediaSource.