favor sort over full regenerateKeys when possible
favor sort over full regenerateKeys when possible

regenerateKeys is more expensive because it allocates,
iterates over the map and typically has a more random input
for sort.Sort

Note: BenchmarkAddAsc is a bit slower because
previously it was buggy (we didn't always add the ts)

taskset --cpu-list 5 go test -bench Benchmark . -benchmem
goos: linux
goarch: amd64
pkg: github.com/grafana/metrictank/mdata/cache
BenchmarkAddAsc         	 1000000	      1119 ns/op	     145 B/op	       3 allocs/op
BenchmarkAddDesc1       	   10000	    706575 ns/op	     191 B/op	       6 allocs/op
BenchmarkAddDesc4       	   10000	    711135 ns/op	     199 B/op	       8 allocs/op
BenchmarkAddDesc64      	    9984	    705479 ns/op	     202 B/op	       8 allocs/op
BenchmarkAddRangeAsc    	 5000000	       427 ns/op	     123 B/op	       0 allocs/op
BenchmarkAddRangeDesc4  	   10000	    196109 ns/op	     131 B/op	       1 allocs/op
BenchmarkAddRangeDesc64 	   99968	    158026 ns/op	     115 B/op	       0 allocs/op
PASS
ok  	github.com/grafana/metrictank/mdata/cache	52.470s

benchcmp post3.txt post4.txt
benchmark                   old ns/op     new ns/op     delta
BenchmarkAddAsc             1053          1119          +6.27%
BenchmarkAddDesc1           1456538       706575        -51.49%
BenchmarkAddDesc4           1452870       711135        -51.05%
BenchmarkAddDesc64          1449829       705479        -51.34%
BenchmarkAddRangeAsc        366           427           +16.67%
BenchmarkAddRangeDesc4      363554        196109        -46.06%
BenchmarkAddRangeDesc64     277954        158026        -43.15%

benchmark                   old allocs     new allocs     delta
BenchmarkAddAsc             3              3              +0.00%
BenchmarkAddDesc1           7              6              -14.29%
BenchmarkAddDesc4           9              8              -11.11%
BenchmarkAddDesc64          9              8              -11.11%
BenchmarkAddRangeAsc        0              0              +0.00%
BenchmarkAddRangeDesc4      1              1              +0.00%
BenchmarkAddRangeDesc64     0              0              +0.00%

benchmark                   old bytes     new bytes     delta
BenchmarkAddAsc             145           145           +0.00%
BenchmarkAddDesc1           21720         191           -99.12%
BenchmarkAddDesc4           21729         199           -99.08%
BenchmarkAddDesc64          21700         202           -99.07%
BenchmarkAddRangeAsc        117           123           +5.13%
BenchmarkAddRangeDesc4      5503          131           -97.62%
BenchmarkAddRangeDesc64     3276          115           -96.49%

replay/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/metrictank,raintank/raintank-metric,replay/metrictank,raintank/metrictank,raintank/raintank-metric