Fix Data Race in TSDB when setting measurement field name

[corylanou]

WARNING: DATA RACE
Write by goroutine 1564:
  runtime.mapassign1()
      /usr/local/Cellar/go/1.5.1/libexec/src/runtime/hashmap.go:411 +0x0
  github.com/influxdb/influxdb/tsdb.(*Shard).createFieldsAndMeasurements()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/shard.go:378 +0x6df
  github.com/influxdb/influxdb/tsdb.(*Shard).WritePoints()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/shard.go:230 +0x640
  github.com/influxdb/influxdb/tsdb.(*Store).WriteToShard()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/store.go:398 +0x1e6
  github.com/influxdb/influxdb/cluster.(*Service).processWriteShardRequest()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/cluster/service.go:214 +0x861
  github.com/influxdb/influxdb/cluster.(*Service).handleConn()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/cluster/service.go:162 +0x5a3
  github.com/influxdb/influxdb/cluster.(*Service).serve.func1()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/cluster/service.go:112 +0x7b

Previous read by goroutine 1286:
  runtime.mapiterinit()
      /usr/local/Cellar/go/1.5.1/libexec/src/runtime/hashmap.go:559 +0x0
  github.com/influxdb/influxdb/tsdb.(*Measurement).FieldNames()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/meta.go:1491 +0x106
  github.com/influxdb/influxdb/tsdb.(*DatabaseIndex).ExpandWildcards()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/meta.go:350 +0x2f5
  github.com/influxdb/influxdb/tsdb.(*DatabaseIndex).RewriteSelectStatement()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/meta.go:309 +0x14a
  github.com/influxdb/influxdb/tsdb.(*RawMapper).Open()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/raw.go:809 +0xd6
  github.com/influxdb/influxdb/tsdb.(*RawExecutor).execute()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/raw.go:69 +0x141

Goroutine 1564 (running) created at:
  github.com/influxdb/influxdb/cluster.(*Service).serve()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/cluster/service.go:113 +0x454

Goroutine 1286 (running) created at:
  github.com/influxdb/influxdb/tsdb.(*RawExecutor).Execute()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/raw.go:59 +0x74
  github.com/influxdb/influxdb/tsdb.(*QueryExecutor).executeStatement()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/query_executor.go:707 +0x104
  github.com/influxdb/influxdb/tsdb.(*QueryExecutor).ExecuteQuery.func1()
      /Users/corylanou/go/src/github.com/influxdb/influxdb/tsdb/query_executor.go:173 +0xbde

[dgnorton]

This line ... https://github.com/influxdb/influxdb/blob/master/tsdb/shard.go#L378

Should be calling https://github.com/influxdb/influxdb/blob/master/tsdb/meta.go#L1480