Speedup wait_for_and_get_block

blockchain
* reduced times in which get_current_block_num is called
* when last_fetched_block_num is given, get_current_block_num is only called when block_number is greater than last_fetched_block_num
storage
* default nodes list enhanced
steemnoderpc
* Service Temporarily Unavailable and Bad Gateway added to exception detection
graphenerpc
* check for Service Temporarily Unavailable and Bad Gateway
* Improved handling of Client returned invalid format. Expected JSON! when output is int
Unit test
test_utils fixed