[Cmd] Set server command seal flag default to false

[DarianShawn]

Description

The default value of seal flag was set to true, which means that a full node has to handle gossip transactions even they don't need it. Then massive transactions flood the node's txpool, and they'll print out huge amount insufficient funds logs .

This PR sets the flag default to false, makes it quiet and restoring calm.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Documentation update

Will upgrade the documentation once it merged.

Additional comments

Associated issue: #156

[codecov-commenter]

Codecov Report

Merging #196 (564da07) into dev (1225fcc) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #196      +/-   ##
==========================================
+ Coverage   51.39%   51.41%   +0.01%     
==========================================
  Files         114      114              
  Lines       16962    16968       +6     
==========================================
+ Hits         8718     8724       +6     
  Misses       7504     7504              
  Partials      740      740              

Impacted Files	Coverage Δ
txpool/txpool.go	69.18% <100.00%> (+0.34%)	⬆️

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[0xcb9ff9]

LGTM