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

Land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/280
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