TracingHandler should ignore active spans

[fzakaria]

in an event loop model, ThreadLocal in ActiveSpanSource does not work and can lead to tricky problems if not explicitly disabled.

• add explicit ignoring of any "active spans" while in the event loop via TracingHandler
• addded unit test

fixes #9

[pavolloffay]

@fzakaria hi sorry I missed this PR. Could you please update the PR, I have merged something which breaks this.

[fzakaria]

@pavolloffay should be clean now.

[pavolloffay]

@fzakaria it seems it is not rebased against master/head as there are references to active span which has been removed.

[pavolloffay]

@fzakaria thanks