This commit enables mailserver cycle logic by default and make a few
changes:
1) Nodes are graylisted instead of being blacklisted for a set amount of
time. The reason is that if we blacklist, any cut in connectivity
might result in long delays before reconnecting, especially on spotty
connections.
2) Fixes an issue on the devp2p server, whereby the node would not
connect to one of the static nodes since all the connection slots
where filled. The fix is a bit inelegant, it always connects to
static nodes, ignoring maxpeers, but it's tricky to get it to work
since the code is clearly not written to select a specific node.
3) Adds support to pinned mailservers
4) Add retries to mailservers requests. It uses a closure for now, I
think we should eventually have a channel etc, but I'd leave that for
later.
Andrea Maria Piana