Add support for zstd multi-threading

[berlincount]

zstd improved the multi-threading code present in pzstd in 1.1.0 and moved it up to the main library. I'd love to be able to use multiple threads - at least when reading zstd compressed streams.

Thanks in advance!

What version of Go are you using (go version)?

go version go1.8.1 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/count"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.8.1/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.8.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/qx/x2xsgzgx5y90lmwwkhs3w91r0000gn/T/go-build691925268=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

[jcftang]

Actually having thread reads and writes would be nice, I'm easily hitting the limits of a single thread where my datasets are in the 10's of gigabytes in size

[bsergean]

Head up that is being looked into in #117

[bsergean]

Just for the writer side of thing.