Enable true async mode

[aleks-f]

Currently, the I/O has to be "serialized" from outside, which defeats the async nature of the I/O to a significant extent; while internally the I/O requests will be going on between multiple peers for every request, the request round-trip must be "outside shielded" (serialized) to prevent concurrent I/O requests.

There are two main reasons for this limitation:

1. On startup, I/O may begin before Session was completely initialized; this has to do with SocketProactor::IOCompletion running in its own thread. Session constructor should wait for the initial discovery I/O to complete before proceeding and allowing kademlia library I/O operations to be issued.

2. When I/O is running concurrently, loading values sometimes fails with "value not found" error; this manifest itself as a timeout, but increasing the timeout value does not help it - for some reason, some peers fail to respond with the sought for value; a "stopgap" that fixes this problem is to treat the timed out peers as new ("uncontacted") on such occurrence, but the true underlying reason for it should be further investigated (this problem does not happen when the loading operations are serialized)