Remove unused records
Unused variable
Perform add + delete in same request

Yokozuna needs to perform both a delete and add for each index
operation.  This is needed to make sure siblings are handled properly
[1].  Currently this is done as two separate HTTP calls.  This patch
combines both into one.

On my MacBook Pro I saw a ~15% improvement in throughput, a 11ms
drop in median latency, and a 15ms drop in 95th latency.

Additionally, I cleaned up the delete API.  There is now just one
function to delete.  A list of `delete_op()` is passed and `yz_solr`
will handle encoding it into JSON.  Added `yz_solr:index/3` so that
delete ops can be sent along with doc additions.

[1]: In the future perhaps we can be smarter about this and add some
type of hook into KV that notifies Yokozuna when a) siblings have been
created or b) when they have been resolved.
Remove unused functions

The boost stuff was a holdover from a copy/paste job I did a long time
ago.  Index boosting is not supported in Yokozuna.