Merge pull request #15327 from asutherland/mail-slice-batching

Bug 862870 - [email] Implement slice batching to reduce/minimize UI reflows, improve responsiveness, sync time. r=asuth
Merge pull request #15337 from asutherland/uplift-slice-batching

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