Allow token holders to change a market without being an LP

[davidsiska-vega]

At the moment to submit a market change proposal you need to meet minimum ELS requirement and minimum vega token holder requirement.

(P) It may be possible that an LP loses keys and yet the ELS means that it is extremely expensive for anyone else to join and meet the minimum ELS requirement. At that point we may end up with a market that e.g. cannot be settled.

The question is whether the criteria for market change proposal should be
a) meet minimum ELS requirement or minimum vega token holder requirement
b) ( minimum ELS requirement and minimum vega token holder requirement for LPs ) or meet minimum vega token requirement for 3rd party i.e. option b) introduces override and the assumption that minimum vega token holder requirement for LPs would be "small" and meet minimum vega token requirement for 3rd party would be "large".

As an aside: for now we can avoid the problem (P) by setting the minimum ELS requirement to 0.

[davidsiska-vega]

Closed by #1768