Global option for skipping cache-clear during updb, pm-enable, pm-disable, ...

Continued from the good discussion at #308.
I think this affects. See https://github.com/drush-ops/drush/blob/master/commands/pm/pm.drush.inc#L1110

`pm-enable` callback calls `drush_get_named_extensions_list()` after enabling the modules. The flow ends up calling `system_rebuild_module_data()`, that has a static.
Also hook_perm is invoked on the new module. I'm not sure if any part around `module_invoke()` has any static, or perhaps the new .module files are not already loaded.