Support setting NumRewriteThreads and NumExpensiveRewriteThreads

Several rewrite driver factory options are duplicated between apache and nginx.  Instead of duplicating this one too, a PSOL change moves them into system/, and this change takes advantage of that move.
options: move global option parsing to system/
Merge pull request #731 from pagespeed/jefftk-thread-config

options: support setting NumRewriteThreads