Refactor couch_replicator_httpc_pool release/supply functionality

BugzID: 41419
Add DNS TTL functionality to couch_replicator_httpc_pool

This feature periodically checks the DNS entry for the target URL. If DNS
returns data that's different from the previous check, all open connections are
gracefully closed in favor of new ones, which will perform a new DNS lookup upon
connection establishment.

BugzID: 41419
Add a code_change clause for DNS TTL additions

BugzID: 41419