Paused replication may report incorrect changes_left

When calculating changes_left for paused replication, XDCR connects to source bucket to get the high-seqnos for the vbuckets. This client is saved for later use:

https://github.com/couchbase/goxdcr/blob/d33173509cfde5fdee067573dfda3404b504b4c8/utils/utils.go#L658

When there are paused replications for other source bucket, the same client is re-used:

https://github.com/couchbase/goxdcr/blob/d33173509cfde5fdee067573dfda3404b504b4c8/utils/utils.go#L646

Causing the other replications to use the high-seqno from the wrong bucket. As a result:

• if all VB mapping happens to be the same between the two buckets and high-seqno can all be obtained, the reported numbers can be wrong and negative changes_left can be reported.
• if VB mapping happens to be different and some VB stats cannot be found, errors like "Can't find high seqno for vbno=855 in stats map. Source topology may have changed" will be reported.

This bug only exist for pre-7.1. In 7.1 the code has changed to use bucket topology service which obtains the high-seqno from the correct bucket, so it is not affected by this bug.