Only evict memory series after they are on disk.

This fixes the problem where samples become temporarily unavailable for
queries while they are being flushed to disk. Although the entire
flushing code could use some major refactoring, I'm explicitly trying to
do the minimal change to fix the problem since there's a whole new
storage implementation in the pipeline.

Change-Id: I0f5393a30b88654c73567456aeaea62f8b3756d9
Speed up disk flushes by removing unnecessary sort.

The first sort in groupByFingerprint already ensures that all resulting sample
lists contain only one fingerprint. We also already assume that all
samples passed into AppendSamples (and thus groupByFingerprint) are
chronologically sorted within each fingerprint.

The extra chronological sort is thus superfluous. Furthermore, this
second sort didn't only sort chronologically, but also compared all
metric fingerprints again (although we already know that we're only
sorting within samples for the same fingerprint). This caused a huge
memory and runtime overhead.

In a heavily loaded real Prometheus, this brought down disk flush times
from ~9 minutes to ~1 minute.

OLD:
BenchmarkLevelDBAppendRepeatingValues   5  331391808 ns/op  44542953 B/op   597788 allocs/op
BenchmarkLevelDBAppendsRepeatingValues  5  329893512 ns/op  46968288 B/op  3104373 allocs/op

NEW:
BenchmarkLevelDBAppendRepeatingValues   5  299298635 ns/op  43329497 B/op   567616 allocs/op
BenchmarkLevelDBAppendsRepeatingValues 20   92204601 ns/op   1779454 B/op    70975 allocs/op

Change-Id: Ie2d8db3569b0102a18010f9e106e391fda7f7883