Fund account spec

[witgaw]

First completed draft of the spec, based on: #1505.

close #866 .

TL;DR

• There’s a way to for parties to pool funds and nominate a key that will be able to allocate those funds within Vega but never outside of it (no possibility of withdrawal for that key).
• There’s a way to change the key managing the funds.
• A mechanism for rewarding the key managing the funds exists (management fee).
• Parties which transferred into the fund account can request the funds (up to the current value of their share of the fund) at any time, however there’s a delay in place to allow the manager to carry out a sensible liquidation of the required portion of fund’s holdings.