Description of race:

1. moveChunk on collection C0 from shard S1 to S0.
2. S1 sends S0 recvChunkCommitStart
3. S0 migrate thread finishes and set active to false.
4. moveChunk on collection C1 from shard S2 to S0.
5. startCommit from #2 proceeds on the loop for active to become false (https://github.com/mongodb/mongo/blob/r2.7.6/src/mongo/s/d_migrate.cpp#L2248)
6. startCommit times out on the condition variable as long as moveChunk in #4 takes more than 30 secs.