Currently bleve batch is build by user goroutine
Then read by bleve gourinte
This is still safe when used correctly
However, Reset() will modify the map, which is now a data race
This fix is to simply make batch.Reset() alloc new maps.
This provides a data-access pattern that can be used safely.
Also, this thread argues that creating a new map may be faster
than trying to reuse an existing one:
https://groups.google.com/d/msg/golang-nuts/UvUm3LA1u8g/jGv_FobNpN0J
Separate but related, I have opted to remove the "unsafe batch"
checking that we did. This was always limited anyway, and now
users of Go 1.6 are just as likely to get a panic from the
runtime for concurrent map access anyway. So, the price paid
by us (additional mutex) is not worth it.
fixes#360 and #260
b8a2fbb887