Add another CNetAddr constructor which accepts boost::asio::ip::address #4301

By merging the independent BoostAsioToCNetAddr() function, this makes things a little bit more straightforward and clearer I think.

Has mentioned in previous PR (https://github.com/bitcoin/bitcoin/pull/4299) which intended to collect feedback.

Signed-off-by: Huang Le 4tarhl@gmail.com

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I intentionally made this a separate function to avoid having the netbase.h header depend on the boost::asio stuff (which slows down compile times a lot, for starters).

In this case, would it be possible to predeclare boost::asio::ip::address so that the header include is not needed there?

@laanwj Wow, I didn't notice that including the asio header would cause a compiling overhead, maybe because the building have already cost a long time;)

It sounds smart to pre-declare the class, will submit a commit soon...