[Backport] Avoid concurrent read and writes to dcp opcode counters

fatal error: concurrent map read and map write

goroutine 1499702 [running]:

runtime.throw(0xda598e, 0x21)

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/panic.go:566 +0x95 fp=0xc430c65058 sp=0xc430c65038

runtime.mapaccess2(0xc71a80, 0xc42f9be900, 0xc430c650db, 0x15bf560, 0x4f1a00)

	/home/couchbase/.cbdepscache/exploded/x86_64/go-1.7.6/go/src/runtime/hashmap.go:340 +0x249 fp=0xc430c650a0 sp=0xc430c65058

github.com/couchbase/eventing/util.SprintDCPCounts(0xc42f9be900, 0xc4455ca000, 0xa5, 0xc430c651b0, 0x4113ff, 0x15a5c20, 0x7fb14bcfa278)

	goproj/src/github.com/couchbase/eventing/util/util.go:139 +0xb7 fp=0xc430c65160 sp=0xc430c650a0

github.com/couchbase/eventing/consumer.(*Consumer).String(0xc42b623400, 0xe415c8, 0xc4410b9800)

	goproj/src/github.com/couchbase/eventing/consumer/v8_consumer.go:551 +0x47 fp=0xc430c65250 sp=0xc430c65160

fmt.(*pp).handleMethods(0xc4410b9800, 0x76, 0x644526c901)