Changing ratePerSecond resets Debt

[smol-ninja]

Problem

If sender changes the ratePerSecond, streamDebtOf returns 0. That means by changing the rps of the stream, the stream loses track of how much the sender owes to the recipient. This should not be allowed.

Potential solutions

1. Does not allow the sender to change rps when debt > 0.
2. Store debt in a separate storage variable.

[smol-ninja]

As discussed over yesterday's call with @andreivladbrg, (1) isn't a good solution as it is very realistic to require changing rps without paying for the accumulated debt.

An example,

• A company increases the salary of an employee which means changing rps but doesn't have funds to pay the debt.

For now, (2) is the only potential solution but I suggest we brainstorm more on this.

[smol-ninja]

Closing this as a duplicate of #64