Make background sync less onerous: Refs bgo#713530

This commit makes three modifications:

  * Introduces some sleep() calls in AccountSynchronizer and
    EmailPrefetcher to avoid thrashing hard when under load
  * Modifies the AccountSynchronizer not to go back so far before
    giving up and to traverse time by three months instead of one
    to locate a folder's epoch email quicker
  * If the max epoch time is reached, just pull in all emails, as
    the server search is apparently not working