Turn the parameter loanId as an indexed parameter in the DepositCollateral event

[jjmr007]

Rationale

It simplifies the searching of all collateral added to a specific loan position

Change

from this:
event DepositCollateral(bytes32 loanId, uint256 depositAmount, uint256 rate);

to this:

event DepositCollateral(bytes32 indexed loanId, uint256 depositAmount, uint256 rate);