Add regression tests for 'loop until op is consumed' bug.

- Most of this is the actual regression test in tiered_test.go.

- Working on that regression tests uncovered problems in
  tiered_test.go that are fixed in this commit.

- The 'op.consumed = false' line added to freelist.go was actually not
  fixing a bug. Instead, there was no bug at all. So this commit
  removes that line again, but adds a regression test to make sure
  that the assumed bug is indeed not there (cf. freelist_test.go).

- Removed more code duplication in operation.go (following the same
  approach as before, i.e. embedding op type A into op type B if
  everything in A is the same as in B with the exception of String()
  and ExtractSample()). (This change make struct literals for ops more
  clunky, but that only affects tests. No code change whatsoever was
  necessary in the actual code after this refactoring.)

- Fix another op leak in tiered.go.

Change-Id: Ia165c52e33290ad4f6aba9c83d92318d4f583517