Add error message if old alert rule tokens are read

Fix missing printed keep_common without grouping.

Never drop a still open head chunk.

Checkpoint fingerprint mappings only upon shutdown

Before, we checkpointed after every newly detected fingerprint
collision, which is not a problem as long as collisions are
rare. However, with a sufficient number of metrics or particular
nature of the data set, there might be a lot of collisions, all to be
detected upon the first set of scrapes, and then the checkpointing
after each detection will take a quite long time (it's O(n²),
essentially).

Since we are rebuilding the fingerprint mapping during crash recovery,
the previous, very conservative approach didn't even buy us
anything. We only ever read from the checkpoint file after a clean
shutdown, so the only time we need to write the checkpoint file is
during a clean shutdown.

Add link to prometheus.io in the README

Add missing Apache license headers + travis check

Prometheus is Apache 2 licensed, and most source files have the
appropriate copyright license header, but some were missing it without
apparent reason. Correct that by adding it.

Also adds a `check_license` target to the Makefile to run the script

Give TestEvictAndLoadChunkDescs more time to actually evict

Add 0.18.0rc1 changelog

Include goversion in build_info metric

Revert "Simplify runtime version code more"

Simplify runtime version code more

It's no longer a build-time injected variable (as the comment above its
block indicates), so move it to the map below.

Use runtime.Version()

Fix potential race in ctx intialization

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