Implement relative complement set operator "unless"

The `unless` set operator can be used to return all vector elements from
the LHS which do not match the elements on the RHS. A use case is to
return all metrics for nodes which do not have a specific role:

    node_load1 unless on(instance) chef_role{role="app"}

Remove unused code leftovers

Use ContainsRune

Work around compiler bug

Benchmarks don't show any significant changes.

Adds holt-winters query function

Beorn7/storage7

Gorilla-style chunks

Pull in fix for and with empty labelsets

Remove old alerting syntax

The chunk encoding was hardcoded there because it mostly doesn't
matter what encoding is chosen in that test. Since type 1 is
battle-hardened enough, I'm switching to type 2 here so that we can
catch unexpected problems as a byproduct. My expectation is that the
chunk encoding doesn't matter anyway, as said, but then "unexpected
problems" contains the word "unexpected".

So far, the last sample in a chunk was saved twice. That's required
for adding more samples as we need to know the last sample added to
add more samples without iterating through the whole chunk. However,
once the last sample was added to the chunk before it's full, there is
no need to save it twice. Thus, the very last sample added to a chunk
can _only_ be saved in the header fields for the last sample. The
chunk has to be identifiable as closed, then. This information has
been added to the flags byte.

This improves fuzz testing in two ways:

(1) More realistic time stamps. So far, the most common case in
practice was very rare in the test: Completely regular increases of
the timestamp.

(2) Verify samples by scanning through the whole relevant section of
the series.

For Gorilla-like chunks, this showed two things:

(1) With more regularly increasing time stamps, BenchmarkFuzz is
essentially as fast as with the traditional chunks:

```
BenchmarkFuzzChunkType0-8              2         972514684 ns/op        83426196 B/op    2500044 allocs/op
BenchmarkFuzzChunkType1-8              2         971478001 ns/op        82874660 B/op    2512364 allocs/op
BenchmarkFuzzChunkType2-8              2         999339453 ns/op        76670636 B/op    2366116 allocs/op
```

(2) There was a bug related to when and how the chunk footer is
overwritten to make use for the last sample. This wasn't exposed by
random access as the last sample of a chunk is retrieved from the
values in the header in that case.

This is not a verbatim implementation of the Gorilla encoding.  First
of all, it could not, even if we wanted, because Prometheus has a
different chunking model (constant size, not constant time).  Second,
this adds a number of changes that improve the encoding in general or
at least for the specific use case of Prometheus (and are partially
only possible in the context of Prometheus). See comments in the code
for details.

Merging what has already been reviewed in other PRs.

Quarantine series upon problem writing to the series file

Rework chunk iterators

Solve the series churn problem...

- Moved returns into the default section of switch statement that can
  only happen then.

- Fix typo.

Instrument scrape pool `sync()`

Instruments:
- duration
- count