MINOR: task: provide 3 task_new_* wrappers to simplify the API

We'll need to improve the API to pass other arguments in the future, so
let's start to adapt better to the current use cases. task_new() is used:
  - 18 times as task_new(tid_bit)
  - 18 times as task_new(MAX_THREADS_MASK)
  - 2 times with a single bit (in a loop)
  - 1 in the debug code that uses a mask

This patch provides 3 new functions to achieve this:
  - task_new_here()     to create a task on the calling thread
  - task_new_anywhere() to create a task to be run anywhere
  - task_new_on()       to create a task to run on a specific thread

The change is trivial and will allow us to later concentrate the
required adaptations to these 3 functions only. It's still possible
to call task_new() if needed but a comment was added to encourage the
use of the new ones instead. The debug code was not changed and still
uses it.
diff --git a/src/connection.c b/src/connection.c
index bf8c60e..a4a8a8b 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -1686,7 +1686,7 @@
 static int allocate_mux_cleanup(void)
 {
 	/* allocates the thread bound mux_stopping_data task */
-	mux_stopping_data[tid].task = task_new(tid_bit);
+	mux_stopping_data[tid].task = task_new_here();
 	if (!mux_stopping_data[tid].task) {
 		ha_alert("Failed to allocate the task for connection cleanup on thread %d.\n", tid);
 		return 0;