Load local msgs and display new msgs more quickly: Closes bgo#713432

This patch is a grab-bag of fixes to get mail onto the screen faster
and report new mail waiting on the server more quickly.

In a nutshell:
  * Adds a NO_DELAY flag to Folder.open_async which indicates that
    background remote connections should initiate ASAP rather than
    wait for a local request that requires remote information.
  * Reduce creation of ImapDB.Folders (which, previously, were
    generated as though "cheap"), which means caching server
    information.  ImapDB now relies on ImapEngine to refresh that
    information on its own.
  * The background search table update is delayed to allow startup
    database tasks priority.
  * Rather than delay selection of a folder 100ms to prevent the user
    from holding down a key or clicking madly, the initial selection
    goes right through, but subsequent ones are delayed.  This may
    also help resolve bug #713468.
  * And the big one: ImapEngine.Account doesn't load local and remote
    folders in parallel at startup, but rather local first, reports
    them to the user, and then loads the remote and pairs the two.
    This gets the UI up and going much more quickly.