use round robin in 'snet treasurer *' and in 'snet client call'

[astroseger]

In the current version user should specify one specific endpoint in all snet treasurer * commands and in 'snet client call' command.

We should read list of endpoints from metadata (group name should be specified by user in case of multiply payment groups ) and we should user round robin to make a call (It seems it is already supported by grpc library)