Allow HttpClient instances to be refreshed periodically

[Mpdreamz]

This PR fixes a longstanding wish to be able to recycle HttpClient instances periodically. We reuse HttpClient/HttpMessageHandler's to prevent TCP leakage but infamously this does not take DNS changes into account.

IHttpClientFactory from Microsoft.Extensions.Http solves this but the nuget package depends on too many things we don't want to depend on OOTB like Dependency Injection. This copies parts of this code (now MIT licensed as of February) so we can reuse the same strategy in the client.

Starting with netstandard2.1 SocketsHttpHander also supports PooledConnectionLifetime this however has a bug in corrolation with MaxConnectionsPerServer. This is fixed in netcoreapp3.0 and up.

The goal is to ship this new RequestDataHttpClientFactory introduced here in all TFM's.

In a follow up commit we'll introduce netcoreapp3.0 and netcoreapp3.1 TFM's that will only use RequestDataHttpClientFactory to create HttpClient instances but won't use its recycling capabilities since that's now handled by PooledConectionLifetime.

[Mpdreamz]

I finally found where to remove travis-ci which was forever pending as we disabled it! It was a required check for master in the branch rules 🎉

[russcam]

LGTM, I left some small comments 👍

[github-actions]

The backport to 6.x failed:

The process 'git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-6.x 6.x
# Navigate to the new working tree
cd .worktrees/backport-6.x
# Create a new branch
git switch --create backport-4673-to-6.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 4c340e87d4fb633593783a812ac867ac56313375
# Push it to GitHub
git push --set-upstream origin backport-4673-to-6.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-6.x

Then, create a pull request where the base branch is 6.x and the compare/head branch is backport-4673-to-6.x.