Cht-core: Improve sms-gateway startup time on well-used installations

[alxndrsn]

As per https://github.com/medic/medic-gateway/issues/19, the event log takes a long time to load when there are a lot of log messages in the database. In turn this means that it can take a while for the GUI to load on starting medic-gateway.

Steps to fixing:

• [x] don't load the event log at startup - this should immediately fix the slow startup issue
• [x] release and share with @bishwas-medic for testing
• [x] load log messages _asynchronously_, and show a spinner while loading - it currently seems that the UI is blocked when loading messages
• [x] re-instate loading of event log at startup now that a nice spinner is shown
• [x] release and share with @bishwas-medic for testing
• [x] introduce use of Loaders for the message lists
• [x] release and share with @bishwas-medic for testing

[estellecomment]

Some class names are shorter than 15 characters, which was so shocking I stopped reviewing.
@bishwas-medic is acceptance-testing, hooray!

[estellecomment]

Apart from shortening names LGTM!

[ngaruko]

Are you able to test this @bishwas-medic ? or shall we sick more insight from @alxndrsn on how to acceptance-test it?

[bishwasBhatta]

Yeah, this feature is already been tested and released. Huge improvement in app load time.