attempting to lock an unint'd mutex may crash on some platforms. And
there is no chance for an unint'd conn to get init'd while shutdown
is occurring, so the unprotected check of struct state is safe.

- fix various connection creation/setup concurrency issues
- use shared connection when always idasserting (similar to ITS#4781)
- reduce the impact of schema mapping when not used (tnx to gprof)
- fix temporary/tainted connection leak in abnormal conditions

(this is preliminary, more work remains)