dcpdrain: Fix user-after-free bug on shutdown

Class MemcachedConnection as stored in vbmap refers to the libevent
object constructed at line 560, so we need to destroy all elements of
vbmap before the libevent eventbase is destroyed.

Change-Id: I012e40e21d85dd1d0f0345c7a9da7124adb08330
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/157349
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@couchbase.com>

programs/dcpdrain/dcpdrain.cc

@@ -729,5 +729,9 @@ int main(int argc, char** argv) {
                   << calculateThroughput(total_bytes, duration.count() / 1000)
                   << ")" << std::endl;
     }
+
+    /// Need to destruct MemcachedConnections before the libevent 'base'
+    /// eventbase as MemcachedConnection refers to the event.
+    vbmap.clear();
     return EXIT_SUCCESS;
 }