adding a transaction table

[djudjuu]

closes #44

• [x] a table to store transactions

• [x] deposit & withdraw functions that adjist user-balances and create a matching transaction in the same mutation

• [x] adjusted helper function that does batch updates to also create batch-transactions

• [x] adding queries to display transaction history (loan, deposit, withdraw related) in lender and borrower dashboards

• [x] tests

• [x] #62 should be reviewed & merged first

context for design decisions

the PK for the tx-table is an int...and not a uuid... I thought it would be nice if we could very easily see which transaction came first and it would be handy to impose an ordering on the txs a user does....a little later I realized that the same could be achieved with created_at...I dont see a downside of having the PK be a tx though....

Open question:

• any input on the fields of the tx-table? I thought we would store any other info on the data-field
• Currently, I just mock the call to financePeer (or any other service provider). I call them inside the function that creates the transactions. This assumes that the FP check will be fast. Alternatively, we could also store the offer on the loan-request... and give financePeer a callback to an endpoint that will then fetch the transactions from the loan-request and apply them... but this seemed to complicated.

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[update in progress for 5841b2c]

[gparuthi]

Looks good to merge once we resolve the id column issue