Optimize compare time using batch interface

[qizhou]

One UX problem of ethfs-cli is the long comparison time if there are multiple local files. To reduce the comparison time, one optimization is done at #17 to issue multiple RPC requests instead of one. However, this approach faces the rate-limiting of RPC servers.

One optimization that can be done is to use a batch interface to get the data hashes of the files in a single RPC call. Suppose we can get 10 data hashes per call, then we can reduce the time to 1/10x without worry about the rate-limiting issue.

[iteyelmp]

fix