Remove positional listing operations from Geary.Folder: Closes #7249

This simplifies the various Folder implementations, removes one (large)
replay queue operation, makes vector expansion _much_ simpler, and
generally makes the Geary API a bit cleaner.

Because the code affected included some of the proposed fixes for
operation.  That plus the vector expansion changes may have some
affect on that ticket.
Smarter account synchronization at startup: Closes #7228

Also removes one annoying debug line from my last commit.
Various bug fixes and optimizations for slow folder/message loading: Refs #7238

Although these fixes don't close the above ticket, they do alleviate many of its
symptoms and generally seems to improve the experience.  It also fixes a couple
of nasty bugs in vector expansion and account synchronization.
Merge branch 'master' into inlinecomp
Merge branch 'master' into inlinecomp