Log syncer

[absolute8511]

This pr will add the support for the raft learner and use learner node to catchup from leader and send logs to remote cluster.

This feature is used to sync data from cluster to cluster in nearly realtime.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e7e3888). Click here to learn what that means.
The diff coverage is 13.62%.

@@            Coverage Diff            @@
##             master      #41   +/-   ##
=========================================
  Coverage          ?   44.66%           
=========================================
  Files             ?      149           
  Lines             ?    21411           
  Branches          ?        0           
=========================================
  Hits              ?     9563           
  Misses            ?    10299           
  Partials          ?     1549

Impacted Files	Coverage Δ
common/util.go	0% <ø> (ø)
node/config.go	100% <ø> (ø)
common/type.go	21.98% <ø> (ø)
node/namespace.go	34.1% <0%> (ø)
node/log_sender.go	0% <0%> (ø)
node/syncer_learner.go	0% <0%> (ø)
common/api_request.go	0% <0%> (ø)
common/stats.go	0% <0%> (ø)
rockredis/t_kv.go	52.55% <0%> (ø)
cluster/pdnode_coord/pd_learner_coord.go	0% <0%> (ø)
... and 24 more

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