Technical Debt - shouldn't use fingerprint function for pseudorandomness because it is not efficient.

[chenchanglew]

the shuffler and smart contract will use the ballots' fingerprints as a seed for generating pseudorandomness for the shuffled proof.
However, the fingerprint method will unmarshal all the encrypted ballots and count their hash of them which is not efficient.
We should come out with a more efficient way to calculate the seed for pseudorandomness.

Where:
services/shuffle/neff/handler.go makeTx()
contracts/evoting/evoting.go shuffleBallots()

Why:
To increase the performance of shuffling and verifying proof.

Target:
add a new function to generate seeds for pseudorandomness.
add unit-test that new function.
all unit tests and integration tests should be passed.