Strict UTF-8 validation fast fail

[zaphoyd]

Presently UTF8 validation is done on a per frame basis. A frame is read and then checked. True fast UTF8 fail would fail as soon as the first bad byte was read. Doing this would require a different setup for reading from ASIO so it will be low priority for now. THis might need to be done in the future anyways, in particular if a streaming api is ever needed.

[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.