use a thread-local variable to get the current running loop

use a thread-local variable to get the current running loop

if we had a method to get the running loop, similar to how
isc_tid() gets the current thread ID, we can simplify loop
and loopmgr initialization.

remove most uses of isc_loop_current() in favor of isc_loop().
in some places where that was the only reason to pass loopmgr,
remove loopmgr from the function parameters.