Fix GC problems in ZookeeperObjectMapperTests

We believe the deadlocks are being caused by startGc()'s notify() call
happening before gcRunnable.run()'s wait() call. This change adds a sentinel
that should prevent a deadlock in that situation (thanks for fixing this
Brandon). This patch also ensures that the GC thread gets unblocked after
every test by calling startGc and waitForGc in the suite teardown.

Change-Id: Ida3bac6af6bfeb29a62f3392bc44b2201dad513b
Signed-off-by: Nicolas Schiper <nicolas@midokura.com>