raft: Avoid allocation when boxing slice in maybeCommit

[nvanbenschoten]

By boxing a heap-allocated slice header instead of the slice
header on the stack, we can avoid an allocation when passing
through the sort.Interface interface.

This was responsible for 26.61% of total allocations in an
experiment with https://github.com/nvanbenschoten/raft-toy.

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

[nvanbenschoten]

cc. @bdarnell

[codecov-io]

Codecov Report

Merging #10679 into master will decrease coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10679      +/-   ##
==========================================
- Coverage   71.63%   71.49%   -0.14%     
==========================================
  Files         394      394              
  Lines       36658    36658              
==========================================
- Hits        26261    26210      -51     
- Misses       8564     8613      +49     
- Partials     1833     1835       +2

Impacted Files	Coverage Δ
raft/raft.go	92.19% <100%> (ø)	⬆️
etcdctl/ctlv2/command/watch_command.go	57.5% <0%> (-22.5%)	⬇️
client/members.go	66.12% <0%> (-19.36%)	⬇️
pkg/logutil/zap_grpc.go	47.61% <0%> (-4.77%)	⬇️
proxy/grpcproxy/watch.go	88.55% <0%> (-4.22%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
etcdserver/util.go	95% <0%> (-3.75%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
clientv3/balancer/balancer.go	84.25% <0%> (-2.37%)	⬇️
etcdserver/api/v3rpc/lease.go	69.31% <0%> (-2.28%)	⬇️
... and 13 more

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