Refactor transaction into batch and snapshot

[weiihann]

Description

This PR removes the Transaction struct and added batch and snapshot which implements the Transaction interface. batch is used for read-write operations while snapshot is used for read-only operations.

Rationale

A transaction can only be a batch or a snapshot, not both at the same time. The following pattern may be unnecessary:

if t.batch != nil {
  ...do something with batch
} else if t.snapshot != nil {
  ...do something with snapshot 
}

It's clearer to just separate batch and snapshot into different structs and use them accordingly.

[codecov]

Codecov Report

Attention: Patch coverage is 74.15730% with 23 lines in your changes missing coverage. Please review.

Project coverage is 78.52%. Comparing base (0c0700c) to head (339d747).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
db/pebble/snapshot.go	46.66%	11 Missing and 5 partials ⚠️
db/pebble/batch.go	86.95%	3 Missing and 3 partials ⚠️
db/pebble/common.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2182      +/-   ##
==========================================
+ Coverage   78.44%   78.52%   +0.07%     
==========================================
  Files         100      102       +2     
  Lines        9191     9205      +14     
==========================================
+ Hits         7210     7228      +18     
+ Misses       1348     1346       -2     
+ Partials      633      631       -2     

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[IronGauntlets]

In general, it looks good to me. I have let some comments