REST API: Fix LedgerStateDelta JSON encoding

[jasonpaulos]

Summary

Removes the Txleases field from JSON state delta representations, as this field cannot be properly JSON encoded.

Based on #6100

Closes #6097

Test Plan

Test coverage added on all 3 endpoints.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 56.26%. Comparing base (23a04c2) to head (2621ac8).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
daemon/algod/api/server/v2/handlers.go	0.00%	11 Missing ⚠️
data/transactions/transaction.go	0.00%	2 Missing ⚠️
libgoal/libgoal.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6106      +/-   ##
==========================================
+ Coverage   56.25%   56.26%   +0.01%     
==========================================
  Files         490      490              
  Lines       69699    69704       +5     
==========================================
+ Hits        39206    39222      +16     
+ Misses      27825    27822       -3     
+ Partials     2668     2660       -8     

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[jannotti]

This looks like a fine fix to make sure the leases can be emitted as valid JSON. On the other hand, I'm struggling to see any value in sending the lease information at all, since it is very explicit already in the transaction (the sender, the lease, and the implied expiration from last valid).
Anyway, I'm ok with this, or zeroing the leases map before creating JSON.