BibCirculation: CERN LDAP improvements

• Improves handling of broken LDAP connections by retrying once more to connect and then giving up.

• Correctly escapes user input before using it in LDAP filters.

• Improves heuristic to discover CERN account corner cases, namely when a user left CERN and she still have a disabled account (identified by the "Primary" employeeType) and a new "ExCern" account.

• Removes no longer needed sleeping points.

• Reduces the number of calls to task_sleep_now_if_required() and task_update_progress().

• Improves progress report wording. (closes #1186)

Tested-by: Tibor Simko <tibor.simko@cern.ch>