Ensure temporal order in streams.

BenchmarkAppendSample.* before this change:

BenchmarkAppendSample1   1000000              1142 ns/op
--- BENCH: BenchmarkAppendSample1
        memory_test.go:81: 1 cycles with 9992.000000 bytes per cycle, totalling 9992
        memory_test.go:81: 100 cycles with 250.399994 bytes per cycle, totalling 25040
        memory_test.go:81: 10000 cycles with 239.428802 bytes per cycle, totalling 2394288
        memory_test.go:81: 1000000 cycles with 255.504684 bytes per cycle, totalling 255504688
BenchmarkAppendSample10   500000              3823 ns/op
--- BENCH: BenchmarkAppendSample10
        memory_test.go:81: 1 cycles with 15536.000000 bytes per cycle, totalling 15536
        memory_test.go:81: 100 cycles with 662.239990 bytes per cycle, totalling 66224
        memory_test.go:81: 10000 cycles with 601.937622 bytes per cycle, totalling 6019376
        memory_test.go:81: 500000 cycles with 598.582764 bytes per cycle, totalling 299291408
BenchmarkAppendSample100           50000             41111 ns/op
--- BENCH: BenchmarkAppendSample100
        memory_test.go:81: 1 cycles with 79824.000000 bytes per cycle, totalling 79824
        memory_test.go:81: 100 cycles with 4924.479980 bytes per cycle, totalling 492448
        memory_test.go:81: 10000 cycles with 4278.019043 bytes per cycle, totalling 42780192
        memory_test.go:81: 50000 cycles with 4275.242676 bytes per cycle, totalling 213762144
BenchmarkAppendSample1000           5000            533933 ns/op
--- BENCH: BenchmarkAppendSample1000
        memory_test.go:81: 1 cycles with 840224.000000 bytes per cycle, totalling 840224
        memory_test.go:81: 100 cycles with 62789.281250 bytes per cycle, totalling 6278928
        memory_test.go:81: 5000 cycles with 55208.601562 bytes per cycle, totalling 276043008
ok      github.com/prometheus/prometheus/storage/metric/tiered  27.828s

BenchmarkAppendSample.* after this change:

BenchmarkAppendSample1   1000000              1109 ns/op
--- BENCH: BenchmarkAppendSample1
        memory_test.go:131: 1 cycles with 9992.000000 bytes per cycle, totalling 9992
        memory_test.go:131: 100 cycles with 250.399994 bytes per cycle, totalling 25040
        memory_test.go:131: 10000 cycles with 239.220795 bytes per cycle, totalling 2392208
        memory_test.go:131: 1000000 cycles with 255.492630 bytes per cycle, totalling 255492624
BenchmarkAppendSample10   500000              3663 ns/op
--- BENCH: BenchmarkAppendSample10
        memory_test.go:131: 1 cycles with 15536.000000 bytes per cycle, totalling 15536
        memory_test.go:131: 100 cycles with 662.239990 bytes per cycle, totalling 66224
        memory_test.go:131: 10000 cycles with 601.889587 bytes per cycle, totalling 6018896
        memory_test.go:131: 500000 cycles with 598.550903 bytes per cycle, totalling 299275472
BenchmarkAppendSample100           50000             40694 ns/op
--- BENCH: BenchmarkAppendSample100
        memory_test.go:131: 1 cycles with 78976.000000 bytes per cycle, totalling 78976
        memory_test.go:131: 100 cycles with 4928.319824 bytes per cycle, totalling 492832
        memory_test.go:131: 10000 cycles with 4277.961426 bytes per cycle, totalling 42779616
        memory_test.go:131: 50000 cycles with 4275.054199 bytes per cycle, totalling 213752720
BenchmarkAppendSample1000           5000            530744 ns/op
--- BENCH: BenchmarkAppendSample1000
        memory_test.go:131: 1 cycles with 842192.000000 bytes per cycle, totalling 842192
        memory_test.go:131: 100 cycles with 62765.441406 bytes per cycle, totalling 6276544
        memory_test.go:131: 5000 cycles with 55209.812500 bytes per cycle, totalling 276049056
ok      github.com/prometheus/prometheus/storage/metric/tiered  27.468s

Change-Id: Idaa339cd83539b5e4391614541a2c3a04002d66d