etcdserver: Reduce the ChanLen of watchResponse,and reduce its memory usage

[cfc4n]

128 seems to be enough.
Sometimes the consumption speed is more than the production speed.

Fixes #11906

[xiang90]

Can you update the comment above the const definition with your reasoning.

[codecov-commenter]

Codecov Report

Merging #11987 into master will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11987      +/-   ##
==========================================
+ Coverage   65.96%   66.50%   +0.53%     
==========================================
  Files         403      403              
  Lines       37306    37320      +14     
==========================================
+ Hits        24608    24818     +210     
+ Misses      11182    10990     -192     
+ Partials     1516     1512       -4     

Impacted Files	Coverage Δ
mvcc/watchable_store.go	92.90% <ø> (+8.86%)	⬆️
auth/options.go	67.50% <0.00%> (-25.00%)	⬇️
client/keys.go	76.88% <0.00%> (-14.58%)	⬇️
pkg/adt/interval_tree.go	85.96% <0.00%> (-1.26%)	⬇️
clientv3/watch.go	90.52% <0.00%> (-1.01%)	⬇️
clientv3/retry_interceptor.go	68.62% <0.00%> (-0.99%)	⬇️
pkg/netutil/netutil.go	68.03% <0.00%> (-0.82%)	⬇️
etcdserver/api/v3rpc/watch.go	83.56% <0.00%> (-0.70%)	⬇️
etcdserver/cluster_util.go	64.88% <0.00%> (-0.45%)	⬇️
etcdserver/v3_server.go	66.47% <0.00%> (-0.20%)	⬇️
... and 21 more

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[cfc4n]

Can you update the comment above the const definition with your reasoning.

Done. thanks for review.