Add anti spam fee for dividend

- Fee is proportional to holder: for each holder the fee is 0.0002 XCP. For instance to pay dividend to 5000 holders, it will cost 1 XCP. I think this fee must be greater than issuance fee, because the potentials "damages" for the network are greater with dividend spam than issuance spam.

- here https://github.com/JahPowerBit/counterpartyd/blob/develop/lib/dividend.py#L145 : 
`if tx['block_index'] >= 327000` is to avoid movements_hash change on mainnet

- here https://github.com/JahPowerBit/counterpartyd/blob/develop/lib/blocks.py#L1144 : 
movements hashes are cleaned on testnet when reparsing. Is to avoid making this manually when there is protocol change.