clientv3: close open watch channel if substream is closing on reconnect

[heyitsanthony]

If substream is closing but outc is still open while reconnecting, then outc
would only be closed once the watch client would connect or once the watch
client is closed. This was leading to deadlocks in the proxy tests. Instead,
close immediately if the context is canceled.

Fixes #7503

[xiang90]

lgtm

[codecov-io]

Codecov Report

Merging #7504 into master will increase coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7504      +/-   ##
==========================================
+ Coverage   70.14%   70.34%   +0.19%     
==========================================
  Files         320      320              
  Lines       26168    26178      +10     
==========================================
+ Hits        18356    18414      +58     
+ Misses       6359     6312      -47     
+ Partials     1453     1452       -1

Impacted Files	Coverage Δ
clientv3/watch.go	90.67% <100%> (+1.11%)	✅
pkg/transport/timeout_conn.go	60% <0%> (-20%)	❌
etcdserver/util.go	94.28% <0%> (-5.72%)	❌
pkg/adt/interval_tree.go	82.43% <0%> (-2.37%)	❌
clientv3/lease.go	91.11% <0%> (-0.89%)	❌
etcdserver/raft.go	87.18% <0%> (-0.54%)	❌
etcdserver/server.go	78.76% <0%> (-0.28%)	❌
proxy/grpcproxy/adapter/watch_client_adapter.go	94.11% <0%> (ø)	✅
etcdserver/api/v3client/v3client.go	100% <0%> (ø)	✅
etcdserver/api/v3rpc/lease.go	95.45% <0%> (ø)	✅
... and 12 more

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