Add two interruption points in dnsseed thread to make shutdown in early ...

...stage quickly

The dnsseed thread is starting by default and takes quick some time, if the user wants to stop the bitcoind by Ctrl-C in command line or a kill in daemon mode when the dnsseed is still working, s/he would have to wait for it to finish all the job, in such situation, an extra 'kill -9' would be expected.

By adding two interruption points just before the time-consuming operations we give the dnsseed thread a chance to realize the user's intention and quit its job much quickly, usually in seconds instead of several minutes or more depends on network situation.

Signed-off-by: Huang Le <4tarhl@gmail.com>