Wait until a cache dump completes instead of waiting for a fixed amount of time

Wait until a cache dump completes instead of waiting for a fixed amount of time

Dumping the cache is an asynchronous operation, so sleeping for a fixed
amount of time after running "rndc dumpdb" is imperfect as dumping cache
contents may take longer than expected on slower machines.  Instead of
always sleeping for 1 second, wait until the "; Dump complete" line
appears in the dump or 10 seconds pass, whichever comes first.