edmPluginRefresh: set DSO to 2000 on Linux

edmPluginRefresh takes around 90 minutes (8 childs) after CMSSW
was built. Set DSO to 2000 to run only with 1 child and lower
required time to scan all plugins.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
edmCheckClassVersion: disable before ROOT run-time is invoked

Currently ROOT6 checksums are different and edmCheckClassVersion is
disabled. More details in ROOT-5727.

Quit before we launch ROOT run-time/load any dictionaries.
Merge pull request #2026 from davidlt/root6-improve-build-time

Improve build time (edmCheckClassVersion and edmPluginRefresh)