Bug 998395 - [Tarako][email] OOM death/email killed from message scrolling with snippet fetching. r=asuth

Targeted uplift of the changes from bug 796474 that relate to block flushing.
Merge pull request #18875 from asutherland/v1.3t-reduce-block-mem

Bug 998395 - [Tarako][email] OOM death/email killed from message scrolling with snippet fetching. r=asuth. a=1.3T+