[Go] Best practice trivial update

• Check defer call error.
  e.g
  https://github.com/thanos-io/thanos/blob/master/pkg/runutil/runutil.go#L138
• For graceful shutdown, exhaust reader before exit.
  e.g
  https://github.com/thanos-io/thanos/blob/master/pkg/runutil/runutil.go#L148
• no Init(), no extern globals (only const allowed)
• Don't use panic (I do use panic for writing to closed channel for graceful shutdown)
• Always measure the results.
  https://vsdmars.blogspot.com/2016/01/c.html
• dereference pointer and write to it is slow (this is basic if coming from C++/C) due to it's really an read and write (not single write)
• Pre-allocating Slices and Maps (same if coming from C++: STL, std::vector is really a new allocate and copy)
• Reuse underline allocated memory from slice:
  slice = slice[:0]
  Slice data structure same as SOO(small object optimization) in std::string,
  pointer to slice, length of current slice, slice mallocaed size / data type (i.e capacity).
• blank _ tips: use for assuring a type implements an interface:
   var _ InterfaceA = TypeA