Add per-node filter queries

Create a custom SearchComponent which allows Yokozuna to perform
per-node filter queries. This removes the need for the `_yz_node` field
and the use of a monolithic filter query. Allowing smaller index size
and less overhead per query.

One microbenchmark showed a 15% increase in query throughput.
Remove unused function
Remove _yz_node field

With the addition of the per-node filter queries there is no longer a
need for the `_yz_node` field.
Fix comment for `_yz_err` field
Type & name refactoring around coverage functions

* Add some types.

* Give, arguably, better names to some functions.
Remove duplicate call
Use short circuit logical OR operator
Merge pull request #271 from basho/rz/perf/per-node-fq

Add per-node filter queries + remove `_yz_node` field