Create MaterializedPackageSets in threads to make pkgset faster.
    
    When modules are used, there are lot of small package sets. These
    package sets have usually less than 500 packages. The createrepo
    part of `MaterializedPackageSet.create` executed for such small
    set of packages takes around 1 second. Most of this time
    the createrepo_c runs in single thread. It does the initialization,
    it writes the XML files, ...
    
    The parts of createrepo which can be run in parallel and therefore
    would use all the CPUs are quite small for very small package sets.
    
    This commit therefore executes multiple threads with
    `MaterializedPackageSet.create` for these very small package sets.
    
    This saves around 40 seconds from pkgset phase for RHEL compose.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>