New BSIP: Free TRANSFERs with POS

```
BSIP:
Title: Free TRANSFERs with POS
Author: [bench]
Status: Draft
Type: Protocol
Created: 2019-09-15
Discussion: https://github.com/bitshares/bsips/issues/212
```

# Abstract
Newer blockchains (EOS, IOTA, NANO ... ) want to improve adoption and offer new use cases with Free Transfers (FT). There are different concepts to prevent spam attacks, but PoS (Proof of Stake) looks like the most convenient one.

# Specifications
- Locking up a certain amount BTS in a smart contract, to allow a certain amount of free transfers during a certain time period.
- One staking account can provide free transfers for other accounts with the Whitelist function.

## FT_parameter
- `ft_rate` (How many BTS are needed per FT and `ft_period`)
- `ft_period` (How long is one period)

## FT_object
- `user_id`
- `ft_locked` (amount of BTS, which are locked for FT)
- `ft_whitelist`

## FT_function
Transfer function needs to check if there is a FT available:

`ft_sum` = counting all FTs from last `ft_period` till now

if `sum_ft` < `ft_locked`/`ft_rate`
`ft_bool` = true
else
`ft_bool` = false

# Copyright
This document is placed in the public domain.

# See also
#191
https://github.com/cryptonomex/graphene/pull/612
https://github.com/bitshares/bitshares-core/issues/186