add namespace::clean to more roles
Rename Bulk & WriteSelector to BulkWrite & BulkWriteView

I renamed Bulk to BulkWrite for greater clarity about what exactly is
being done in bulk and to make the next change sound better.

I renamed WriteSelector to BulkWriteView in anticipation of being
consistent with the fluent API which will have Collection and
CollectionView.  The idea is that a "FooView" constrains the Foo with a
document selection and only allows operations that are relevant after
find().
Rename _Selector role to _View

This role will be consumed by views to provide the query document
and an op queue for dispatching writes
rename _OpQueue to _WriteQueue

Also renames various methods accordingly for clarity.
tidy code
refactor BulkWriteView into roles

The behavior is decomposed into several distinct roles.  These
will also support CollectionView when the fluent API is implemented.