WebSocketClient only accepted timeouts as (time_t sec, time_t usec), while
ClientImpl has taken std::chrono::duration overloads for its read, write and
connection timeouts for a long time. Add the same three overloads, forwarding
through the existing detail::duration_to_sec_and_usec helper so the split
matches ClientImpl exactly.
The template bodies go above the first split.py BORDER, next to the class, so
that the .h/.cc split keeps them in the header where instantiation needs them.
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