Cht-core: Improve application performance for high-utilization CHPs

[ghost]

In testing, @sglangevin has been experiencing major application performance problems with a newly-replicated set of documents from a high-performing CHP (total database size: 3430 documents). Operations that fetch data were taking on the order of 10-15 seconds, leading to lags in the UI and general un-usability.

In analysis, we noticed that allDocs was outperforming query for this IndexedDB database, which led us to a major performance problem in PouchDB's cacheUpdateRequired (part of a changeset that is now reverted).

Tasks:

• [x] Get a patch in to the v2.8.x branch for immediate testing
• [x] Figure out whether production is being impacted by this performance problem
• [x] Produce a point release with the patch for the 2.7.x series, or decide it's unnecessary (e.g. we're confident CHPs won't hit the problem, or LG is being upgraded directly to 2.8.0)

[garethbowen]

Patch: https://github.com/medic/medic-webapp/commit/153aec454f3148265b8a4b34dfd0246668adfa91

[garethbowen]

My understanding is everyone on 2.7.x will get on to 2.8.x soon so the backport to 2.7.x shouldn't be necessary.

[ghost]

Performance increase confirmed.

v2.7.2:

v2.8.0-b6:

[sglangevin]

This was AT'ed already. Moving to ready.