Refactor: move devnull from global variable to the stack

in drain_notification_channel ( http://src.couchbase.org/source/xref/trunk/kv_engine/daemon/thread.cc#249 ) we use a global variable devnull which is a blackhole (we just want to discard the data). This cause thread sanitizer to warn about a race (as we write to it from multiple threads without locking), so we've got a suppression rule in place http://src.couchbase.org/source/xref/trunk/tlm/tsan.suppressions#54 .

We can just refactor the code to keep the variable on the stack (probably smaller to avoid blowing the stack) and remove the suppression.