lib: use parallel compression

[iaguis]

Makes conversion of on-disk images about 3 times faster on my system
(Intel(R) Core(TM) i7-6500U CPU @ 2.50GHz).

[jonboulle]

Huh. Sounds good to me if it works.

[iaguis]

$ docker2aci -version
docker2aci version v0.9.3
appc version 0.7.4
$ time docker2aci redis.docker 
Extracting c94ee45783f5
Extracting 866d119d4baf
Extracting 4e7a0cf03ae5
Extracting 73c5693a4e4d
Extracting 10243e8f6c09
Extracting 8e56edf9e1aa
Extracting deb7bc1f6941
Extracting 2f37973f55bb
Extracting 6da3a1a99559
Extracting 8198ae410841
Extracting d233a90c81b9
Extracting de404d2b7d55
Extracting 92c4bb455223
Extracting 3d31a0a27d50
Extracting eff82d92b8fc
Extracting 86cf6e2d2939
Extracting d95ec32bf524

Converted volumes:
    name: "volume-data", path: "/data", readOnly: false

Converted ports:
    name: "6379-tcp", protocol: "tcp", port: 6379, count: 1, socketActivated: false

Generated ACI(s):
redis-latest.aci

real    0m27.972s
user    0m27.170s
sys     0m1.677s
$ docker2aci -version
docker2aci version v0.9.3-11-gf7c8fdf
appc version 0.8.2
$ time docker2aci redis.docker 
Extracting c94ee45783f5
Extracting 866d119d4baf
Extracting 4e7a0cf03ae5
Extracting 73c5693a4e4d
Extracting 10243e8f6c09
Extracting 8e56edf9e1aa
Extracting deb7bc1f6941
Extracting 2f37973f55bb
Extracting 6da3a1a99559
Extracting 8198ae410841
Extracting d233a90c81b9
Extracting de404d2b7d55
Extracting 92c4bb455223
Extracting 3d31a0a27d50
Extracting eff82d92b8fc
Extracting 86cf6e2d2939
Extracting d95ec32bf524

Converted volumes:
    name: "volume-data", path: "/data", readOnly: false

Converted ports:
    name: "6379-tcp", protocol: "tcp", port: 6379, count: 1, socketActivated: false

Generated ACI(s):
redis-latest.aci

real    0m8.304s
user    0m14.137s
sys     0m0.713s