readme: document building off of release-1.5.27.1-beta
threading: revert "Use ngx_thread_system in NgxRewriteDriverFactory."

This reverts commit e20088e58d036606d65fbb2bffd10f155e69a2e5.

The change had:

    SystemRewriteDriverFactory(ngx_thread_system_ = new NgxThreadSystem)

This is probably undefined behavior.  I'm going to redo this change to make
NgxRewriteDriverFactory take an NgxThreadSystem* argument instead.