chore: Rename prs to trk

[ystkfujii]

Performed the following steps.

• replace prs with trk

  ./ -name '*.go' -type f -print0 | xargs -0 sed -i -e "s/prs/trk/g"
  git restore tracker/state.go

• delete TODO comment

  // TODO(tbg): rename to trk.

  see: https://github.com/etcd-io/raft/pull/114/files#diff-d7ffa919352e4a9c13fdb7b7fe6fa5302de94a745b90ae80155605e995ef5f11L351

[ahrtr]

cc @pavelkalinnikov to double check

[pav-kv]

Seems fine.