Refactor db size command

[weiihann]

Description

This PR modifies the db size command to also check for leftover key-value pairs in the database. It also fixes an upper bound bug as the UpperBound iterator option is exclusive, not inclusive.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.38%. Comparing base (f5dc02c) to head (1ecb18d).

Files with missing lines	Patch %	Lines
cmd/juno/dbcmd.go	50.00%	4 Missing and 2 partials ⚠️
db/pebble/db.go	80.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2223      +/-   ##
==========================================
- Coverage   78.39%   78.38%   -0.01%     
==========================================
  Files         104      104              
  Lines        9316     9338      +22     
==========================================
+ Hits         7303     7320      +17     
- Misses       1365     1368       +3     
- Partials      648      650       +2     

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