Add a new method to store data synchronously

[shankari]

This is intended for foreground use only, primarily to synchronously save
user inputs in case we need it for server-specific services (e.g. generating a
suggestion) or in case we want to ensure that the data is saved before the app
is uninstalled.

Since it is used in the foreground, users can be interactively notified if the
save failed.

In order to reduce abuse for sending background sensed data, this can accept one entry at a time.

For client integrators, as you can see, this is similar to /profile/update, but the data is under the_entry instead of update_doc

Testing done:

We have 76 entries

In [16]: edb.get_usercache_db().find({"metadata.key": "manual/mode_confirm"}).count()
Out[16]: 76

We save one more

In [17]: test_input = {
    ...:   "user": "test_mode_purpose",
    ...:   "the_entry" : {
    ...:      "metadata": {"key": "manual/mode_confirm", "write_ts": 12345678, "type": "
    ...: message"},
    ...:      "data": {"start_ts": 1234, "end_ts": 5678, "label": "pogo_sticking"}
    ...:   }
    ...: }

In [18]: requests.post("http://localhost:8080/usercache/putone", data=json.dumps(test_in
    ...: put), headers=headers)
Out[18]: <Response [200]>

START 2019-05-08 19:17:35.633951 POST /usercache/putone
END 2019-05-08 19:17:35.649836 POST /usercache/putone 49e72fc4-f1d8-4f4d-9666-57dc597d1133 0.015433073043823242

We now have 77 entries

In [19]: edb.get_usercache_db().find({"metadata.key": "manual/mode_confirm"}).count()
Out[19]: 77

and the new one is the right one

In [20]: edb.get_usercache_db().find({"metadata.key": "manual/mode_confirm", "data.label
    ...: ": "pogo_sticking"}).count()
Out[20]: 1

In [21]: edb.get_usercache_db().find_one({"metadata.key": "manual/mode_confirm", "data.l
    ...: abel": "pogo_sticking"})
Out[21]:
{'_id': ObjectId('5cd38dbfbe04ee70f576335d'),
 'data': {'end_ts': 5678, 'label': 'pogo_sticking', 'start_ts': 1234},
 'metadata': {'key': 'manual/mode_confirm',
  'type': 'message',
  'write_ts': 12345678},
 'user_id': UUID('49e72fc4-f1d8-4f4d-9666-57dc597d1133')}

This also fixes e-mission/e-mission-docs#351 in a
more principled way.

[AmplabJenkins]

Merged build finished. Test FAILed.

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Test FAILed.

[shankari]

jenkins test this please

[shankari]

jenkins, test this please

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Merged build finished. Test PASSed.

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Test PASSed.
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