store: support for <> != <val> matcher is not implemented after upgrade to V0.3.0

[arnitolog]

Thanos, Prometheus and Golang version used

• Thanos: all components 0.3.0
• Prometheus: 2.7.1

What happened
After upgrading query and store components from 0.2.1 to 0.3.0 we started seeing below errors for queries which use "not-equal" (!=) binary operator:

receive series: rpc error: code = Aborted desc = fetch series for block 01D3EFDYWZ6WX3C1SEHF3ESA95: expanded matching posting: match labels: support for <> != <val> matcher is not implemented; empty matcher for label name mode

What you expected to happen
query component returns the correct value

How to reproduce it (as minimally and precisely as possible):
any query with != (not-equal) binary operator. My, in particular, looks like:
node_cpu_seconds_total{mode!="idle"}
Full logs to relevant components

Anything else we need to know

[bwplotka]

Fix in progress.

[bwplotka]

#839