pull in dns_seeds and is_routable.
WM.
remove archaeological version of get_header.
catch EOFError.
expose unpack_var_int.
getheaders.

get_next_500 => get_next_n:
  getheaders can do blocks of 2000.
cmd_getheaders: new method.
cmd_getdata: do not spawn self.send_blocks - out of order uncool.
new_entry: increment length by # of outputs.
add option to use getblocks rather than getheaders.