m3db defaults streamXConcurrency options to 0 if a subset is specified

[Nick Lanham]

Running m3db v1.1.0

We had a config with:

"db":
  "bootstrap":
    "peers":
      "streamPersistShardConcurrency": 4
      "streamShardConcurrency": 4`

but no streamPersistShardFlushConcurrency. This meant that it defaulted to 0, and after doing the first stage of bootstrapping shards, that is until getting 4 messages of the form:

{"level":"info","ts":X,"msg":"peer bootstrapped shard","bootstrapper":"peers","shard":X,"numSeries":X,"blockStart":X}

m3db would just go idle and make no further progress.

Setting streamPersistShardFlushConcurrency to 4 fixes things.

[Gibbs Cullen]

@nicklan -- we will be implementing this change, and will follow back up on this issue shortly.