feat: add tracker for buffer unaggregated fees

[gusinacio]

Closes #289

[coveralls]

Pull Request Test Coverage Report for Build 11127757675

Details

• 155 of 162 (95.68%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.6%) to 68.902%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tap-agent/src/agent/sender_account.rs	29	36	80.56%

Files with Coverage Reduction	New Missed Lines	%
tap-agent/src/agent/sender_account.rs	1	93.8%

Totals
Change from base Build 10967317486:	0.6%
Covered Lines:	4347
Relevant Lines:	6309

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💛 - Coveralls

[gusinacio]

This update mostly solves the problem but it's not 100% accurate. We have a system where we trigger a complete re-calculation of unnagregated receipts. When this happens, newer receipts that are in the queue would already be on the counter and that's why we check if id > unaggregated_fees.last_id so we don't have a double counter on that.

The problem is that current system for buffer needs them to be called via add instead of update to be added to the buffer counter. That means that information about what is in buffer is lost under higher loads.

For this to be updated accordingly, tracker would need to have an internal last_id meaning that the value is added to the buffer, but not to the total tracker.

This is not a big issue since the update is usually called in rav requests which reduces total amount, but this issue must be tracked in case problems happen.

[carlosvdr]

lgtm