Added a ledger chunk class to manage the Transactions in a single chunk

[msftsettiy]

This PR contains changes to split the Ledger class into two. The Ledger class is responsible for iterating over the ledger chunks and the Ledgerchunk class is responsible for iterating over the Transactions in a chunk.

[achamayou]

Thanks @msftsettiy, that change looks good!

[jumaffre]

/azp run

[ghost]

master@13311 aka 20200929.38 vs master ewma over 50 builds from 12741 to 13311

[jumaffre]

@msftsettiy Changes look good - thanks!

Could you format ledger.py with black? Also, the governance_history_test seems to be failing:

38: Traceback (most recent call last):
38:   File "/mnt/build/2/s/tests/governance_history.py", line 124, in <module>
38:     run(args)
38:   File "/mnt/build/2/s/tests/governance_history.py", line 72, in run
38:     ) = count_governance_operations(ledger)
38:   File "/mnt/build/2/s/tests/governance_history.py", line 23, in count_governance_operations
38:     tables = tr.get_public_domain().get_tables()
38: AttributeError: 'Ledgerchunk' object has no attribute 'get_public_domain'
29/51 Test #38: governance_history_test ..........***Failed   11.66 sec

It is the only test we have that uses the ledger.py file. The fix should be pretty trivial but please reach out if you need a hand.

[jumaffre]

/azp run

[achamayou]

/azp run

[achamayou]

@msftsettiy could you run './scripts/ci-checks.sh -f' to apply canonical formatting to your changes please? The PR is otherwise ready to merge.

[eddyashton]

/azp run