[WriteALogPackage] flush before call return readers, binary search

[hhow09]

1. proglog/WriteALogPackage/internal/log/log.go

   Line 200 in f6dae5f

   func (l *Log) Reader() io.Reader {

should we flush the store before return reader just like what Read() did ? Otherwise records might still in buffer.

2. proglog/WriteALogPackage/internal/log/log.go

   Line 99 in f6dae5f

   func (l *Log) Read(off uint64) (*api.Record, error) {

can use binary search to search offset since segments are sorted.