This is both a refactor and perf boost, with more room for improvement, key changes:
- Removes StreamBuffer, hoisted into McapStreamReader
- Reuses Reader and DataView class across parse calls, only resetting them when necessary (e.g: append in McapStreamData)
- Splits `parseRecord` into small scoped parsing functions, this itself is perf neutral (slightly positive) but facilitates future monomorphic fast paths
- Moves offsets tracking into Reader which is cleaner and faster