Memory leak in http 5.3.6

[edsko]

The below is a memory profile of a client using http2-5.3.5 who makes repeated connections to a server and then has a short interaction with the server on that connection before closing it again and starting again:

With http-5.3.6 this profile looks like this instead:

I will try to figure out whta's happening and submit a PR.

[edsko]

(The leak is still present in 5.3.7 also.)

[edsko]

Confirmed that this leak is gone as of #156, and no new memory leak was introduced in the time-manager-0.2 refactoring (see also #155 (comment)).