Get running total workload size from work queue

[tsutsu]

Given the accompanying change to couch_index_updater, the total workload processed in an index update is now unbounded. To ensure that the couch_task_status remains meaningful in cases where work continues to be found after the first couch_index_updater update-loop, couch_mrview_updater has been modified to dynamically find the total amount of work discovered and queued so far and use it to update the couch_task_status. To do this, it uses a new counter which has been introduced to couch_work_queue (total_in) in an accompanying PR.

COUCHDB-2955
BugzID: 14036

[kxepal]

N.B.: This requires apache/couchdb-couch#147 and apache/couchdb-couch-index#14