Fix the deadlock.

We cannot call WebConnection.Close() from the timer thread while
holding the ServicePoint.SyncRoot lock because that method could
block on the WebConnection lock.
Merge branch 'baulig-http-fixes-7.0.4' of github.com:mono/mono into baulig-http-fixes-7.0.4

Conflicts:
	mcs/class/System/System.Net/ServicePoint.cs
	mcs/class/System/System.Net/WebConnectionGroup.cs