Disallow concurrent backups; consult transport requestBackupTime()

* We now check for in-progress backups when asked to perform one, and don't
  bother firing off another one concurrently (nor do we adjust the scheduling;
  after all, someone asked to do a backup "now" and we're doing one, so we are
  in line with expectations).  We also defer backup passes while a restore is
  in flight, and abort attempts to perform a restore during a backup pass.

* When a backup attempt fails, we now ask the transport how far in the future we
  should put our retry.  Previously we were simply not bothering to consult with
  the transport, so we would wind up retrying backup while network connectivity
  was known to be down, etc.

Change-Id: Ic7758010b74e06e90c50d46b7b0dd01b17af7c90