Add should_fetch to lockless_queue so we can conditionally fetch

For logging, we want to be able to only fetch the next message if it is
time to do so.  This adds a callback which is provided all the context
information at the right point to decide if we actually fetch or not.
Next step is to actually expose it through EventLoop.

Change-Id: Ic15eedc46116f8acfcb5a80ed551e89e9d6b7fa8
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/ipc_lib/BUILD b/aos/ipc_lib/BUILD
index b6c2a89..147e0ba 100644
--- a/aos/ipc_lib/BUILD
+++ b/aos/ipc_lib/BUILD
@@ -209,6 +209,7 @@
         "//aos:configuration",
         "//aos:realtime",
         "//aos:uuid",
+        "//aos/events:context",
         "//aos/time",
         "//aos/util:compiler_memory_barrier",
         "@com_github_google_glog//:glog",