With repeated experiments, the TinyDB observer only logs the last one
I was trying to automatically run an experiment several times and log the results using the tinydb observer. The code of the experiment follows here (test.py):
```python
import numpy as np
from sacred import Experiment
from sacred.observers import TinyDbObserver

ex = Experiment('normal_experiment')
ex.observers.append(TinyDbObserver.create('sacred_results'))

@ex.config
def cfg():
  mu = 0.0
  std = 1.0

@ex.automain
def run(mu, std):
    return np.random.normal(loc=mu, scale=std)
```

I run it 3 times with slightly different parameters like so:
```python
from test import ex

ex.run()
ex.run(config_updates={'mu': 0.5})
ex.run(config_updates={'mu': 15})
```

When I inspect the database afterwards, there is only 1 entry corresponding to the last run.

I have also found that when I recreate the observer every time like so:
```python
from test import ex
from sacred.observers import TinyDbObserver

ex.run()
del ex.observers[0]
ex.observers.append(TinyDbObserver.create('sacred_results'))

ex.run(config_updates={'mu': 0.5})
del ex.observers[0]
ex.observers.append(TinyDbObserver.create('sacred_results'))

ex.run(config_updates={'mu': 15})
```
all 3 entries are logged correctly.

I am not sure whether this is a bug, or whether I am perhaps not using the API as intended.

diff --git a/sacred/observers/tinydb_hashfs.py b/sacred/observers/tinydb_hashfs.py
index f4896c1..6d8c5eb 100644
--- a/sacred/observers/tinydb_hashfs.py
+++ b/sacred/observers/tinydb_hashfs.py
@@ -127,6 +127,7 @@ class TinyDbObserver(RunObserver):
 
     def started_event(self, ex_info, command, host_info, start_time, config,
                       meta_info, _id):
+        self.db_run_id = None
 
         self.run_entry = {
             'experiment': dict(ex_info),
['sacred/observers/tinydb_hashfs.py']