[geary: 8/8] Merge branch 'wip/713530-background-sync'. Fixes Bug 713530.

[Michael Gratton <mjog src gnome org>]

commit b1d3d494bc1cf33b64a1214bc493b42ea5f4c76a
Merge: 6c6737d 7c5d89b
Author: Michael James Gratton <mike vee net>
Date:   Sat Dec 2 12:18:57 2017 +1100

    Merge branch 'wip/713530-background-sync'. Fixes Bug 713530.

 src/engine/api/geary-folder.vala                   |  117 +++++--
 src/engine/imap-db/imap-db-folder.vala             |  329 ++++++++++----------
 .../imap-engine-account-synchronizer.vala          |  138 +++++----
 .../imap-engine/imap-engine-email-prefetcher.vala  |    8 +-
 .../imap-engine-abstract-list-email.vala           |  139 +++++----
 .../replay-ops/imap-engine-list-email-by-id.vala   |    9 +-
 src/engine/imap/api/imap-folder.vala               |   38 ++-
 7 files changed, 443 insertions(+), 335 deletions(-)
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