The NetworkManager usage was mostly for low connection counts and this
made defaults selection easy.
With more usages it is important to allow the NWM-connection to be more
configurable about memory usage and leaner in general.
This changes the headers-buffers (used to create envelopes) to not be per
connection anymore but per thread using the tread_local keyword.
This changes the ring-buffers to become configurable using
NetworkConnections::setMessageQueueSizes().
Also removing some include statements where they were not really needed
in the P2PNet lib.
We reuse the NetworkManager lower level code in order to connect
to the Bitcoin P2P network.
This implements the basics for anyone wanting to be a player on
this network.
TomZ