Drop multithreading in bootstrap

[palday]

With improvements/changes in BLAS multithreading, Julia-level threads are competing with the BLAS threads and we don't see any improvement in most situations with use_threads=true. In fact, we sometimes see performance degradation. So let's drop multithreading for the bootstrap and simplify that code. We should instead develop an example for the docs showing how to optionally parallelize using Distributed for people with access to clusters / large computer.

• Failures in threaded_replicate test #640
• Deprecate mulithreading in bootstrap #674

NB: This doesn't prevent us from adding threading to various custom linear algebra methods, if those prove worthwhile. That said, I think that should be an internal performance optimization and not expose any changes in the external API.