Upload packages in parallel

[czoido]

Changelog: Feature: New argument of conan upload command --parallel to upload packages using multithreading.
Docs: conan-io/docs#1250

• Uses a ThreadPool to parallelize upload across recipes and packages upload.
• Fixes output so that output messages and progress bars do not overlap.
• If --parallel option is present, the upload will use 8 threads to upload and use only one thread in the case it is missing.

Closes #3452

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[memsharded]

I have added a mutex to avoid problems while loading hooks.

This PR is failing, not because of the PR itself but because of the ConanApiAuthManager layer, that is not designed for concurrency

I have opened #5957, which must be fixed first before this PR can be merged.

[memsharded]

I have merged develop and pushed. Then testing a little bit:

• seems more robust, all seems working fine.
• The final output messages become useless when parallel, it is impossible to know what has been uploaded. Lots of interleaved Uploading conan_package.tgz completed [10943.20k] messages. I guess they should be replaced with something like Uploaded pkg/0.1:ID conan_package.tgz [...]
• I haven't been able to make it fail with Artifactory, but it is also true that the deduplication of Artifactory has made difficult to repeat the testing several times.

[memsharded]

Good!
Make sure to do a last round of manual testing (py27, py36, a couple of platforms) to check everything is good, and can be merged.
Create a UX issue to improve:

• the text of the downloaded messages, something that is useful as a log of what exactly has been uploaded
• In platforms like CI or cmake.conan, the output is a few lines of blank text, which reads weird. Try if that output can be improved too.

[lasote]

@czoido create the issue and merge.