This allows UDP receive buffer size to be increased to avoid potential
packet loss result from receive buffer overrun.
Another related flag is --io_cache_size. buffer_size in UDP options
defines the UDP buffer size of the underlying system while
io_cache_size defines the size of the internal circular buffer managed
by Shaka Packager.
Closes#411
Change-Id: I57c843a88f13da546417dadc2a78df2bee0a00f3
KongQun Yang