fix(logspout): bump packet size to 1048576

By default, linux's socket receivebuffer is set to 256. The kernel
doubles this value to allow space for bookkeeping overhead when it is
set, therefore our maximum packet size is only 512 bytes. Bumping to a
much larger number will prevent an issue like this happening again.
Using SetWriteBuffer sets the operating system's transmit buffer
associated with this connection, and SetReadBuffer sets the operating
system's receive buffer.
ref(logspout): use UDP conn