Ensure that all extracted samples are added to view.

The current behavior only adds those samples to the view that are extracted by
the last pass of the last processed op and throws other ones away. This is a
bug. We need to append all samples that are extracted by each op pass.

This also makes view.appendSamples() take an array of samples.
extractValuesAroundTime() code simplification.
Make getValuesAtIntervalOp consume all chunk data in one pass.

This is mainly a small performance improvement, since we skip past the last
extracted time immediately if it was also the last sample in the chunk, instead
of trying to extract non-existent values before the chunk end again and again
and only gradually approaching the end of the chunk.
Merge pull request #268 from prometheus/fix/value-extraction

Interval value extraction fixes and optimizations.