To prevent duplicated running of checkUpstream function at the same
time, upstream monitor uses a boolean to report whether the upstream is
checking. If this boolean is true, then other calls after the first one
will be returned immediately.
However, checkUpstream does not set this boolean to false when it
finishes, thus all future calls to checkUpstream won't be run, causing
the upstream is marked as down forever.
Fixing this by ensuring the boolean is reset once checkUpstream done.
While at it, also guarding all upstream monitor operations with a mutex,
ensuring there's no race condition between marking upstream state.
5c24acd952