Strict UTF-8 validation fast fail about websocketpp

[zaphoyd]

This has been implemented for a bit. websocketpp::DEFAULT_READ_THRESHOLD controls the size of the read/validation batches. Smaller values catch UTF8 errors sooner but involve more ASIO overhead. 1 = fail on first byte, 512 is a pretty good default. Note: a value of <10 is required to pass the AB fast fail test as of version 0.5.0. This may change in the future as values this small are inefficient.

[zaphoyd]

also, endpoint::set_read_threshold(size_t) can be used to set that value at runtime for all future connections, connection::set_read_threshold(size_t) can set it for an individual connection.