Reduce and then remove task exclusive mode

This is (umbrella) issue to track the removal of task exclusive mode which (with proper design) should not be necessary. The exclusive mode is used in:

• named itself when:
  • (re-)loading configuration
  • loading zones
  • shutting down
  • enabling/disabling DNSSEC validation via rndc
  • flushing cache
  • deleting TSIG?
  • dumping changes to dynamic zones via rndc
  • freezing/thawing zone
  • something with adding/removing zones via rndc
  • destroying managed-keys database
  • setting TCP timeouts #WTF?
  • configuring serve-stale via rndc
• dns_adb
• dns_catz via catz_addmodzone_taskaction(), catz_delzone_taskaction()
• dns_dt
• ns_clientmgr
• ns_interfacemgr