Bug 862870 - [email] Implement slice batching to reduce/minimize UI reflows, improve responsiveness, sync time. r=asuth, a=blocking-b2g=1.3+ Uplift

Uplift:
https://github.com/mozilla-b2g/gaia/pull/15327
https://github.com/mozilla-b2g/gaia/commit/1c58606fb725112904ab206475ffaaf5158c5881