text analysis now moved out of index write lock onto goroutine

1. text analysis is now done before the write lock is acquired
2. there is now a pool of analysis workers
3. the size of this pool is configurable
4. this allows for documents in a batch to be analyzed concurrently

as a part of benchmarking these changes i've also introduce a new
null storage implementation.  this should never be used, as it
does not actualy build an index.  it does however let us go
through all the normal indexing machinery, without incuring
any indexing I/O.  this is very helpful in measuring improvements
made to the text analsysis pipeline, which are often overshadowed
by indexing times in benchmarks actually building an index.