Speed up "update-specs"

[miparnisari]

Issue #, if available: #1249

Description of changes:
Speed up --update-specs by using concurrent.futures and spreading the work among several threads.

# before
MacBook@ ~/workspace-sync/cfn-python-lint (master)*$ time cfn-lint --update-specs

real	0m36.339s
user	0m11.634s
sys	0m0.636s

# after
MacBook@ ~/workspace-sync/cfn-python-lint (update-specs-fast)*$ time cfn-lint --update-specs

real	0m13.914s
user	0m12.440s
sys	0m1.590s

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[PatMyron]

cfn-lint --update-specs still generating src/cfnlint/data/CloudSpecs/ changes like the ones here:

pip3 install -e . --user
cfn-lint --update-specs
git status

[miparnisari]

Cancelling in favor of #1383