Fix the wbcache dependency chain issue.

[sanebay]

Anytime a split of node happens, we create a dependency chain group1 right node -> left node -> parent node. If a split happens again on any of these nodes or they were dirty due to a write, we copy the buffer and create a new chain group2. This group2 should be flushed to disk after the group1. so we mark group2's dependency as group1. We append the group2 to any node which is modified in group1. Add the dependency to any dirty buffer which has modification. Either left or parent can be modified, add to the end of it.

[codecov-commenter]

Codecov Report

Attention: 118 lines in your changes are missing coverage. Please review.

Comparison is base (a0291e4) 61.87% compared to head (4ea4bad) 62.19%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/lib/device/journal_vdev.cpp	62.61%	74 Missing and 6 partials ⚠️
src/lib/device/device_manager.cpp	80.00%	23 Missing and 4 partials ⚠️
src/lib/logstore/log_stream.cpp	68.75%	4 Missing and 1 partial ⚠️
src/lib/device/virtual_dev.cpp	77.77%	2 Missing ⚠️
src/lib/device/journal_vdev.hpp	90.90%	1 Missing ⚠️
src/lib/device/physical_dev.hpp	50.00%	0 Missing and 1 partial ⚠️
src/lib/logstore/log_dev.cpp	90.90%	1 Missing ⚠️
src/lib/logstore/log_store_family.cpp	66.66%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
+ Coverage   61.87%   62.19%   +0.31%     
==========================================
  Files         110      110              
  Lines        8725     8999     +274     
  Branches     1122     1159      +37     
==========================================
+ Hits         5399     5597     +198     
- Misses       2804     2862      +58     
- Partials      522      540      +18     

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