BibHarvest: better handling of timeouts
- When OAI source times out, keep trying a few times. (closes #853)
- Adds CFG_OAI_FAILED_HARVESTING_STOP_QUEUE variable to let admin configure if failed harvesting should stop BibSched queue in cases where the execution can be fully recovered at next run of the task. CFG_OAI_FAILED_HARVESTING_EMAILS_ADMIN also lets admin configure if an email should be sent in these cases. (addresses #856)