Add missing FindNode prototype to net.h

Also make the argument a const std::string & instead of pass-by-value.
Add comment about never updating nTimeOffset past 199 samples

Refer to issue #4521 for details.

Conflicts:
	src/timedata.cpp
Avoid querying DNS seeds, if we have open connections.

The goal is to increase independence and privacy.

Conflicts:
	src/init.cpp
	src/net.cpp