MDL-11521 fixed problems when creating contexts and aprents does not exist yet

MDL-11521 fixed problems when creating contexts and aprents does not exist yet
MDL-11527 fixed warning from $CFG->defaultuserroleid when installing and upgrading
+ removed reusing of previous contexts in has_capability() and get_context_instance()  because it was hiding critical errors when context was false, not specified, etc.
+ tweaked roles defaults - faster and defaults are applied when roles exist
+ SYSCONTEXTID may be specified in config.php to eliminate 1 db query
+ static context cache is now reset after rebuilding of context paths
+ path field in context table is now nullable - pg compatibility requirement
+ other minor fixes