Optimize label matcher memory and runtime behavior.

This optimizes the runtime and memory allocation behavior for label matchers
other than type "Equal". Instead of creating a new set for every union of
fingerprints, this simply adds new fingerprints to the existing set to achieve
the same effect.

The current behavior made a production Prometheus unresponsive when running a
NotEqual match against the "instance" label (a label with high value
cardinality).

BEFORE:
BenchmarkGetFingerprintsForNotEqualMatcher        10   170430297 ns/op  39229944 B/op    40709 allocs/op

AFTER:
BenchmarkGetFingerprintsForNotEqualMatcher      5000      706260 ns/op    217717 B/op     1116 allocs/op

Change-Id: Ifd78e81e7dfbf5d7249e50ad1903a5d9c42c347a