Fix new logging approach to work in release mode.

Method inlining was breaking the approach taken (traversing the
stack!) to determine which logger to instantiate.

As long as the method is only ever called from within class constructors
avoiding inlining of it should mean it always works :/

(see https://github.com/thinktecture/Thinktecture.IdentityServer.v3/issues/139#issuecomment-46446997 for the fact I've noted this change might well be rejected :) !)

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