refactor: made the main Lua VM state creation and initialization code detached from the configuration loading phase.
refactor: introduce the ngx_http_lua_get_main_lua_state function to abstract away where the global lua state is stored.
bugfix: turning off lua_code_cache could lead to memory issues (segfaults and LuaJIT assertion failures) when Lua libraries using LuaJIT FFI were loaded. now we always create clean separate lua VM instances for every Nginx request when the Lua code cache is disabled. thanks Ron Gomes for the report.