async.c: included macro SEARCH_PAIR from commit 5b1a8f67
async.c: included macro SEARCH_PAIR from commit 5b1a8f67

Commit 5b1a8f67 added two macros LL_SEARCH_PAIR and LL_SEARCH_PAIR2
to utlist.h which is maintained at [1]. To simplify updates of this
file, the macros have been moved into async.c as this is the only
place where they are used. To avoid confusion with linked list macros
from utlist.h, the prefix "LL_" has been removed from the custom
macros.

[1] https://github.com/troydhanson/uthash

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap