Blocking Executors and maxPendingPersists, oh my!

- Execs.newBlockingSingleThreaded can now accept capacity = 0.
- Changed default maxPendingPersists from 2 to 0.
- Fixed serde of maxPendingPersists in RealtimeIndexTasks.
Merge pull request #419 from metamx/mpp

Blocking Executors and maxPendingPersists, oh my!