Correct total byte counters for server.

Previously the getnettotals was just looping through all of the currently
connected peers to sum the byte counts and returning that.  However, the
intention of the getnettotals RPC is to get all bytes since the server was
started, so this logic was not correct.

This commit modifies the code to keep an atomic counter on the server for
bytes read/written and has each peer update the server counters as well as
the per-peer counters.