CBQE-1096 add active tasks stats related post condition checker

This post condition checker works for indexer, bucket_compaction,
view_compaction. For example before rebalance we can have a phase
to check the initial index building.

                "18" :
                {
                    "name" : "finish index building before rebalance",
                    "workload" : [{"spec" : "g:100,coq:defaultph2keys,ops:0",
                                  "conditions" : {"post": {"type":"indexer", "target":"_design/d1", "conditions": "progress > 99"}}}]
                }

From the cbsystest.py command line, syntax is:

python cbsystest.py run workload --create 100 --ops 100 --cluster default --postcondition "indexer:_design/d1, progress > 99"

Change-Id: Iccb006f8265c17c7c3d4c8371686907e4c103a60
Reviewed-on: http://review.couchbase.org/25331
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: buildbot <build@couchbase.com>