Data race on memdopmap drain/add

A previous pull request from the community (https://github.com/couchbase/gocbcore/pull/3) has highlighted a race that occurs when a node is destroyed during operations. This is quite reliably reproduced by running a large number of (sequential) KV ops and killing the node during the process. After ~55s the race occurs.

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WARNING: DATA RACE
Read at 0x00c4201d2300 by goroutine 12:
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdOpMap).Add()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdopmap.go:52 +0x186}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdClient).SendRequest()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdclient.go:98 +0x6b}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdPipelineClient).ioLoop()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdpipelineclient.go:109 +0x306}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdPipelineClient).Run()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdpipelineclient.go:155 +0x30b}}

Previous write at 0x00c4201d2300 by goroutine 16:
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdOpMap).Drain()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdopmap.go:139 +0x10f}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdClient).run.func2()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdclient.go:285 +0x7ae}}

Goroutine 12 (running) created at:
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdPipeline).StartClients()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdpipeline.go:70 +0xa9}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdClientMux).Start()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdclientmux.go:48 +0x7f}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*Agent).applyConfig()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agentrouting.go:250 +0x99e}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*Agent).connect()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agent.go:813 +0xb22}}
{{ gopkg.in/couchbase/gocbcore%2ev7.createAgent()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agent.go:639 +0xfa9}}
{{ gopkg.in/couchbase/gocbcore%2ev7.CreateAgent()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agent.go:514 +0xb0}}
{{ gopkg.in/couchbase/gocb%2ev1.createBucket()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocb.v1/bucket.go:39 +0x3f}}
{{ gopkg.in/couchbase/gocb%2ev1.(*Cluster).openBucket()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocb.v1/cluster.go:263 +0x295}}
{{ gopkg.in/couchbase/gocb%2ev1.(*Cluster).OpenBucket()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocb.v1/cluster.go:277 +0x72}}
{{ main.main()}}
{{ /Users/charlesdixon/dev/go-workspace/src/sandbox/main.go:86 +0x22c}}

Goroutine 16 (finished) created at:
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdClient).run()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdclient.go:226 +0x112}}
{{ gopkg.in/couchbase/gocbcore%2ev7.newMemdClient()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdclient.go:48 +0x1ab}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*Agent).dialMemdClient()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agentrouting.go:43 +0x2ab}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*Agent).slowDialMemdClient()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agentrouting.go:166 +0x1c7}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*Agent).(gopkg.in/couchbase/gocbcore%2ev7.slowDialMemdClient)-fm()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/agentrouting.go:221 +0x59}}
{{ gopkg.in/couchbase/gocbcore%2ev7.newMemdClientMux.func1()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdclientmux.go:22 +0x52}}
{{ gopkg.in/couchbase/gocbcore%2ev7.(*memdPipelineClient).Run()}}
{{ /Users/charlesdixon/dev/go-workspace/src/gopkg.in/couchbase/gocbcore.v7/memdpipelineclient.go:148 +0x263}}