Backport Bitcoin PR#7906: net: prerequisites for p2p encapsulation ch…

…anges (#1521)

* net: don't import std namespace

This file is about to be broken up into chunks and moved around. Drop the
namespace now rather than requiring other files to use it.

* net: remove unused set

* net: use the exposed GetNodeSignals() rather than g_signals directly

* net: make Ban/Unban/ClearBan functionality consistent

- Ban/Unban/ClearBan call uiInterface.BannedListChanged() as necessary
- Ban/Unban/ClearBan sync to disk if the operation is user-invoked
- Mark node for disconnection automatically when banning
- Lock cs_vNodes while setting disconnected
- Don't spin in a tight loop while setting disconnected

* net: No need to export DumpBanlist