Fix value as string on response for /transaction/pool by sender

[sstanculeanu]

Reasoning behind the pull request

• invalid value returned
  

Proposed changes

• use value as string

Testing procedure

• standard system test with some txs in pool and call /transaction/pool?by-sender=erd1...&fields=value and make sure value is a valid string, not as in the above picture

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

[gabi-vuls]

System test passed .

@@ Log scanner @@

fix_value_on_transaction_pool_route

================================================================================

• Known Warnings 7
• New Warnings 1
• Known Errors 0
• New Errors 1
• Panics 0
  ================================================================================
• block hash does not match 1726
• miniblocks does not match 0
• miniblock hash does not match 0
• receipts hash missmatch 0
• wrong nonce in block 781
  ================================================================================
• No jailed nodes on the thestnet
  ================================================================================