Add secondary RPC to SafeEthClient

[Hyodar]

One direct improvement to SafeEthClient stability should be having two RPCs at once, and we can specify the behaviors in each situation. In my opinion:

• Any call should be made on at most both RPCs (1st first, 2nd if the 1st failed)
• Subscriptions should be duplicated and share the same sink with some LRU management (e.g. if both receive a log, we just send one, if only one receives a log, we send one as well)