Reuse httpclient to avoid leaking in .net core 2.1

[cijothomas]

Fix Issue #594 .
On .Net Core 2.1, creating new httpclient for every request has led to leaking of httpConnection related objects/timer eventually leading to high CPU. More details in below issue
microsoft/ApplicationInsights-aspnetcore#690

Didn't find an easy way to unit test this, so relying on existing tests of channel.

• I ran Unit Tests locally.

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