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.

lib/isc/include/isc/loop.h

@@ -27,6 +27,16 @@ typedef void (*isc_job_cb)(void *);
 
 ISC_LANG_BEGINDECLS
 
+/*%<
+ * Returns the current running loop.
+ */
+
+extern thread_local isc_loop_t *isc__loop_local;
+
+static inline isc_loop_t *
+isc_loop(void) {
+	return (isc__loop_local);
+}
 void
 isc_loopmgr_create(isc_mem_t *mctx, uint32_t nloops, isc_loopmgr_t **loopmgrp);
 /*%<