Add backup mongo URLs

[JoranAngevaare]

What is the problem / what does the code in this PR do
Use Chicago and xenon1t-daq.lngs.infn.it as backup mongo instances for fried-rice.

Use fried-rice always as the first default as that is accessible outside dali

Can you briefly describe how it works?
Try to connect to fried-rice, if it doesn't use a backup_mongo_url

Can you give a minimal working example (or illustrate with a figure)?

import strax
import straxen

RunDB = <see PR>

st = strax.Context(
        config=straxen.contexts.xnt_common_config,
        **straxen.contexts.common_opts)
username = straxen.get_secret('rundb_username')
password = straxen.get_secret('rundb_password')
st.storage = [RunDB(
        readonly=True,
        runid_field='number',
        new_data_path='./strax_data',
)
             ]
st.storage[0].client['xenonnt'].command('ping')

Difference with #212
Rather than having all tree in a single URL we split it to a to stage initialization. Otherwise if not on dali it cannot find the primary host.

[ershockley]

If we want to go this route it makes more sense to use the Chicago mirror as the default IMO -- it's much closer to midway/dali. I also don't get why this is a preferred method to #212?

[JoranAngevaare]

Evan and I had a chat, here is an example showing that the approach in #212 fails because mongo cannot decide on who is primary. This PR (213) is a patch rather than the solution to the problem that is shown in the appended gist.