Recover if crashing between posting root msg and adding self to group

[Powersource]

for #40

[Powersource]

Added a failing test

ok 19 there is 1 used group feed and 1 we created now with a root message but not ourselves as a member
not ok 20 there are still only 2 group feeds after creating another group. the unused one got used by create()
  ---
    operator: equal
    expected: 2
    actual:   3
    at: <anonymous> (/home/me/prj/ssb/ssb-tribes2/test/create.test.js:243:25)

[Powersource]

aah a test is failing

[Powersource]

@staltz @mixmix fixed tests!

[staltz]

Looks good to merge. The implementation is getting trickier, and maybe eventually it'll make sense to create easy APIs to query groups and our group feeds, but no need to anticipate that right now