as there is no further use of isc_task in BIND, this commit removes it, along with isc_taskmgr, isc_event, and all other related types.
functions that accepted taskmgr as a parameter have been cleaned up. as a result of this change, some functions can no longer fail, so they've been changed to type void, and their callers have been updated accordingly.
the tasks table has been removed from the statistics channel and the stats version has been updated. dns_dyndbctx has been changed to reference the loopmgr instead of taskmgr, and DNS_DYNDB_VERSION has been udpated as well.
Closes #3797 (closed)