Bug 838843 - [email] email search does not update as headers are modifie...

...d or deleted, allows user to still see and fail to display deleted messages

The work in progress PR is raised for https://bugzilla.mozilla.org/show_bug.cgi?id=838843

1. Unable to cherry-pick from yurenju github due to conflicts, have done manual merge.
2. Added code for updating search headers on modtags.

Currently facing intermittent errors in updating flags/move or delete. Possible error, not able to generate idx at 
https://github.com/psingapati/gaia-email-libs-and-more/blob/Bug_838843_SearcSlice_Flags_Delete_Move_Operations/data/lib/mailapi/searchfilter.js#L674

Please review and give your comments