num_retries improved

Steemnoderpc
* self.errror_cnt_call used instead of local variable doRetryCount
Graphenerpc
* num_retries_call added for setting a max number of retries on a rpc call
* self.n_urls added for getting the number of given nodes
* sleep on retry removed, when switching over to the next node
self.error_cnt_call used for counting rpc call retries
rpcutils
* sleep_and_check_retries improved
Unit tests
num_retries added
test_golos reduced