feat(storagenode): remove package internal/batchlet

[ijsong]

What this PR does

This PR removes the package internal/batchlet that is kind of small batch. Thus
batch in Append RPC message is not splitted any more.

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.06%. Comparing base (daf76e3) to head (fc7d431).

Files with missing lines	Patch %	Lines
internal/storagenode/logstream/append.go	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                                        Coverage Diff                                         @@
##           refactor_logstream_deprecate_old_replicatetask_pool_management     #967      +/-   ##
==================================================================================================
- Coverage                                                           80.09%   80.06%   -0.04%     
==================================================================================================
  Files                                                                 179      178       -1     
  Lines                                                               21592    21509      -83     
==================================================================================================
- Hits                                                                17295    17221      -74     
+ Misses                                                               3502     3500       -2     
+ Partials                                                              795      788       -7     

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