light.js RPC call priotization

[amaury1093]

For simplicity, the balanceOf$.setFrequency() function has been removed, but it would be an idea to put it back.

The idea is:

• right now, each RpcObservable has its frequency fixed (i.e. balanceOf$ calls eth_getBalance on each new block).
• now let's imagine a dapp that makes a huge amount of rpc calls. It would be nice to prioritize these calls.
  • At least console.warn the dapp developer that there are too many rpc calls at the same time
  • Let the dapp developer choose the frequency of each RpcObservable (e.g. some eth_getBalance can be called on every 2 blocks)