Add concurrent stream decompression of snappy/s2 streams

Fixes #397

```
e:\gopath\src\github.com\klauspost\compress\s2\cmd\s2d (master -> klauspost)
λ s2d -bench=3 -cpu=1 enwik10.snappy
Reading enwik10.snappy...
Decompressing... 4340323220 -> 10000000000 [230.40%]; 8.681s, 1098.6MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 8.744s, 1090.7MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 8.711s, 1094.8MB/s

e:\gopath\src\github.com\klauspost\compress\s2\cmd\s2d (master -> klauspost)
λ s2d -bench=3 -cpu=2 enwik10.snappy
Reading enwik10.snappy...
Decompressing... 4340323220 -> 10000000000 [230.40%]; 5.259s, 1813.4MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 5.241s, 1819.8MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 5.255s, 1814.7MB/s

e:\gopath\src\github.com\klauspost\compress\s2\cmd\s2d (master -> klauspost)
λ s2d -bench=3 -cpu=4 enwik10.snappy
Reading enwik10.snappy...
Decompressing... 4340323220 -> 10000000000 [230.40%]; 2.63s, 3625.6MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 2.646s, 3604.8MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 2.644s, 3606.5MB/s

e:\gopath\src\github.com\klauspost\compress\s2\cmd\s2d (master -> klauspost)
λ s2d -bench=3 -cpu=8 enwik10.snappy
Reading enwik10.snappy...
Decompressing... 4340323220 -> 10000000000 [230.40%]; 1.38s, 6910.6MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 1.387s, 6873.4MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 1.387s, 6875.8MB/s

e:\gopath\src\github.com\klauspost\compress\s2\cmd\s2d (master -> klauspost)
λ s2d -bench=3 -cpu=16 enwik10.snappy
Reading enwik10.snappy...
Decompressing... 4340323220 -> 10000000000 [230.40%]; 894ms, 10668.5MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 882ms, 10818.2MB/s
Decompressing... 4340323220 -> 10000000000 [230.40%]; 903ms, 10558.8MB/s
```