Optimization: avoid lazy loading /members call when all members are known already

[bwindels]

Once synapse reports the correct number of invited and joined members again (right now it does but we're about to change this and give an estimate to speed up lazy loading in synapse, this will be accurate again once work on denormalizing room state has been implemented in synapse) in a room when LL members, we could avoid /members requests if the amount of members we already know about is the same as the invited + joined count reported in the summary.

[t3chguy]

This could in theory save a lot of API hits for encrypted DMs, as know the member list for those as it fits within the lazy loading summary