NuGet exhausted sockets

[wli3]

NuGet had this problem with mac #2004 and now it is happening for RHEL7.2 and CentOS7.1

This is blocking CLI check in for 2.1.3xx

Issue on CLI and investigation https://github.com/dotnet/cli/issues/8972

[wli3]

Did nuget keep creating httpclient? A new httpclient instance will hold socket.

[wli3]

@rohit21agrawal @rrelyea

cc @livarcocc

[wli3]

The issue is mitigated by change ulimit on CI. I can confirm this is the problem. But we need to fix the root cause. Ideally there should not be more sockets used than the entry count of nuget config. The limit of 4096 should be enough.