Configure the proxy so he could handle better a lot of requests

I changed three things that makes the proxy more stable while getting a lot of requests (specially during CouchDB replications).

* I set up an agent on the proxy object to avoid recreating a new one for each connection.
* I use the graceful-fs library to handle better situation where too many file descriptors are opened. It monkey patches the standard fs library.
* I added a dirty logging of the error to be sure that the error is displayed all the time.

There is less error while I perform syncing, but I still experience problems while syncing Cozy with my laptop. I'm still investigating it.