Gracefully handle cbauth errors from dcp client

panic: CBAuth database is stale: last reason: EOF

goroutine 8 [running]:

github.com/couchbase/indexing/secondary/common.(*CbAuthHandler).GetCredentials(0xc085aa0540, 0x0, 0x0, 0x0, 0x0)

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/common/util.go:220 +0xdb

github.com/couchbase/indexing/secondary/dcp.maybeAddAuth(0xc08b4ff2b0, 0x2885280, 0xc085aa0540)

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/dcp/pools.go:250 +0x45

github.com/couchbase/indexing/secondary/dcp.queryRestAPI(0xc08b915500, 0xc08b2ecba0, 0x58, 0x2885280, 0xc085aa0540, 0xe3ce80, 0xc0883b6900, 0x0, 0x0)

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/dcp/pools.go:274 +0x214

github.com/couchbase/indexing/secondary/dcp.(*Client).parseURLResponse(0xc08c90a268, 0xc08b2ecba0, 0x58, 0xe3ce80, 0xc0883b6900, 0x0, 0x0)

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/dcp/pools.go:388 +0x7c

github.com/couchbase/indexing/secondary/dcp.(*Bucket).Refresh(0xc0820c5e00, 0x0, 0x0)

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/dcp/pools.go:489 +0xd6

github.com/couchbase/indexing/secondary/common.func·007()

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/common/dcp_seqno.go:287 +0xe9

github.com/couchbase/indexing/secondary/common.pollForDeletedBuckets()

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/common/dcp_seqno.go:292 +0xac

created by github.com/couchbase/indexing/secondary/common.init·1

        C:/Jenkins/workspace/sherlock-windows/couchbase/goproj/src/github.com/couchbase/indexing/secondary/common/dcp_seqno.go:23 +0xa0