maybeStartBuildsForSlave after insubstantiating

[djmitche]

(also handle the deferred from self.insubstantiate elsewhere)

[djmitche]

(this easily cherry-picks onto the eight branch)

[codecov-io]

Current coverage is 84.79%

Merging #1831 into master will decrease coverage by -0.01% as of 2c0c800

@@            master   #1831   diff @@
======================================
  Files          344     344       
  Stmts        31831   31833     +2
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit          26993   26994     +1
  Partial          0       0       
- Missed        4838    4839     +1

Review entire Coverage Diff as of 2c0c800

---

Uncovered Suggestions

1. +0.11% via ...ot/status/builder.py#381...414
2. +0.09% via ...ot/status/builder.py#453...481
3. +0.09% via ...dbot/changes/mail.py#478...505
4. See 7 more...

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[jaredgrubb]

LGTM!