Can support custom value for default_max_transfer_size?

[ghost]

@Invincibl-e commented on Apr 20, 2018, 5:11 PM UTC:

I met a problem, must send the specified size of packets at once... Now I can only change the default_max_transfer_size from the source. Can you set a macro?

This issue was moved by chriskohlhoff from boostorg/asio#99.