fix: Deadlock issues on posting loan repayment (backport #264)

[mergify]

Resolved some deadlock issues while posting multiple parallel payments against single loans

• Converted multiple queries on loan doctype into single query
• Removed unnecessary indexes from loan demand doctype

  ---

  This is an automatic backport of pull request fix: Deadlock issues on posting loan repayment #264 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   lending/loan_management/doctype/loan/test_loan.py

Unmerged paths:
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	both modified:   lending/loan_management/doctype/loan_repayment/loan_repayment.py

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