Add instruction cache

[tonistiigi]

First implementation of instruction cache using the precalculation method. Content based hashing will need updates better to implement copy before that. This method should always apply to all vertexes.

» ./buildkit --target containerd  | buildctl build
INFO[0000] tracing logs to /tmp/buildctl474429996
[+] Building 14.0s (24/24) FINISHED
 => docker-image://docker.io/library/alpine:latest                                                    0.0s
 => => resolve docker.io/library/alpine:latest                                                        0.5s
 => docker-image://docker.io/library/golang:1.8-alpine                                                0.0s
 => => resolve docker.io/library/golang:1.8-alpine                                                    0.4s
 => CACHED apk add --no-cache g++ linux-headers                                                       0.0s
 => CACHED apk add --no-cache git make                                                                0.0s
 => CACHED apk add --no-cache git                                                                     0.0s
 => CACHED git clone https://github.com/moby/buildkit.git /go/src/github.com/moby/buildkit            0.0s
 => CACHED git checkout -q master                                                                     0.0s
 => CACHED cp -a /src/go /dest/                                                                       0.0s
 => go build -o /bin/buildd-containerd -tags containerd ./cmd/buildd                                  8.4s
 => CACHED go build -o /bin/buildctl ./cmd/buildctl                                                   0.0s
 => CACHED cp -a /src/bin/buildctl /dest/bin/                                                         0.0s
 => CACHED git clone https://github.com/opencontainers/runc.git /go/src/github.com/opencontainers/ru  0.0s
 => apk add --no-cache btrfs-progs-dev                                                                4.8s
 => git clone https://github.com/containerd/containerd.git /go/src/github.com/containerd/containerd   3.1s
 => CACHED git checkout -q v1.0.0-rc3                                                                 0.0s
 => CACHED cp -a /src/go /dest/                                                                       0.0s
 => CACHED go build -o /usr/bin/runc ./                                                               0.0s
 => CACHED cp -a /src/usr/bin/runc /dest/bin/                                                         0.0s
 => git checkout -q master                                                                            0.2s
 => cp -a /src/go /dest/                                                                              0.3s
 => make bin/containerd                                                                               8.5s
 => cp -a /src/go/src/github.com/containerd/containerd/bin/containerd /dest/bin/                      0.1s
 => cp -a /src/bin/buildd-containerd /dest/bin/                                                       0.1s
 => ls -l /bin

@AkihiroSuda

[AkihiroSuda]

LGTM 👍

[AkihiroSuda]

It might be good to create separate boltdb bucket like _instructioncache for ease of debugging, but it can be later