[Rhythm] Block builder performance improvement

[mdisibio]

What this PR does:
More low-hanging improvements to block builder:

• Convert live traces to store byte slices to reduce GC scanning of complex objects. It is now a generic type and the generators still use tempopb structs. Ingesters could now make use of this too.
• Parallelize proto unmarshal
• Repoll prealloc slices
• Query partition end offset and use it to avoid the last empty poll (this is mostly for benchmarking and testing, it is minor for actual performance)

This is about 20% improvement in a larger primarily single tenant cluster.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]