Fix async flush logic and flaky unit test

[yzang2019]

Describe your changes and provide context

This PR will fix 3 issues:

1. Flaky unit test is caused by async writes not committing latest version in time before db is closed, resulting in the recovery process being skipped since latestVersion = 0
2. Async write is only enabled properly when useDedicatedChangelog=true, however, we want all async writes are going through the extra WAL file to avoid data loss so that we can recover during initilization.
3. Fix closing order, we should close the channel first, wait for all pending changes to be flushed to DB and then closing the WAL

Testing performed to validate your change

[codecov]

Codecov Report

❌ Patch coverage is 65.38462% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.73%. Comparing base (c3ee918) to head (2f2afd7).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
ss/store.go	11.76%	12 Missing and 3 partials ⚠️
ss/pebbledb/db.go	85.71%	2 Missing and 1 partial ⚠️

❌ Your patch status has failed because the patch coverage (65.38%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (40.73%) is below the target coverage (60.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   40.00%   40.73%   +0.73%     
==========================================
  Files          59       59              
  Lines        7510     7534      +24     
==========================================
+ Hits         3004     3069      +65     
+ Misses       4161     4120      -41     
  Partials      345      345              

Files with missing lines	Coverage Δ
config/config.go	0.00% <ø> (ø)
ss/rocksdb/db.go	56.60% <100.00%> (+11.58%)	⬆️
ss/pebbledb/db.go	60.69% <85.71%> (+5.25%)	⬆️
ss/store.go	36.76% <11.76%> (-11.13%)	⬇️

... and 1 file with indirect coverage changes

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[masih]

A small note that if Close is called twice for whatever reason close will panic when the channel is already closed. Ditto for rocksdb.

[yzang2019]

Make sense, will add a check to make sure it's not called twice