Fix panic when droppping measurement while writing to it concurrently

[jwilder]

There is a race when writing points and dropping measurements where the fields are removed from the index while points are attempting to use them for encoding.

Produces this panic in 0.9.1-rc1

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x8 pc=0x19c625]

goroutine 2744 [running]:
github.com/influxdb/influxdb/tsdb.(*Shard).WritePoints(0xc2081ac000, 0xc21b450000, 0x1388, 0x1800, 0x0, 0x0)
    /Users/jason/go/src/github.com/influxdb/influxdb/tsdb/shard.go:246 +0x415
github.com/influxdb/influxdb/tsdb.(*Store).WriteToShard(0xc208034be0, 0x6, 0xc21b450000, 0x1388, 0x1800, 0x0, 0x0)
    /Users/jason/go/src/github.com/influxdb/influxdb/tsdb/store.go:278 +0xf7
github.com/influxdb/influxdb/cluster.func·002(0x6, 0x1, 0xc21b450000, 0x1388, 0x1800)
    /Users/jason/go/src/github.com/influxdb/influxdb/cluster/points_writer.go:241 +0xfc
created by github.com/influxdb/influxdb/cluster.(*PointsWriter).writeToShard
    /Users/jason/go/src/github.com/influxdb/influxdb/cluster/points_writer.go:271 +0x30b

Fixes #2608

[otoolep]

Minor: I haven't seen us do this before i.e. reference bug numbers. I think the comment is pretty good as it is.

[jwilder]

Removed the bug number reference.

[otoolep]

Seems reasonable.+1