remove a changelog double entry
- [REMOVED] `PrefixMatcher` is considered unused so was removed.
- [CLEANUP] `Options.WALFlushInterval` is removed as it wasn't used anywhere.
- [FEATURE] Add new `LiveReader` to WAL pacakge. Added to allow live tailing of a WAL segment, used by Prometheus Remote Write after refactor. The main difference between the new reader and the existing `Reader` is that for `LiveReader` a call to `Next()` that returns false does not mean that there will never be more data to read.
- - [CHANGE] Empty blocks are not written during compaction [#374](https://github.com/prometheus/tsdb/pull/374)
## 0.3.1
- [BUGFIX] Fixed most windows test and some actual bugs for unclosed file readers.
