Issue:
fix(router_env): resolve/remove `FIXME`'s and redundent files

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Enhancement

## Description
This PR is regarding the `FIXME`'s and `TODO`'s resolution for the crate `router_env`
<!-- Describe your changes in detail -->


### Additional Changes

- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!-- 
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
This PR is part of the chain of micro PR's focused on removing todo
1. #136 
2. #139 
3. #144 
4. #151 
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## How did you test it?
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## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed submitted code

Resolution:
<think>
PR Context:
- Title: fix: remove and resolve `FIXME`s and `TODO`s from db module
- Merged: 2022-12-20T12:25:25Z
- Files changed: 22 files
- Commits: 9 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Enhancement

## Description
This change addresses `todo!()` in the `MockDb` interface implementation and group them under issue #172 . It fixes and removes other `FIXME`s from the db folder.
<!-- Describe your changes in detail -->


### Additional Changes

- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!-- 
Provide links to the files with corr
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** fix: remove and resolve `FIXME`s and `TODO`s from db module (#174)
Files to modify:


**crates/common_utils/src/lib.rs**
  Modify existing code

**crates/router/src/core/errors.rs**
  Modify existing code

**crates/router/src/db/address.rs**
  Modify existing code

**crates/router/src/db/configs.rs**
  Modify existing code

**crates/router/src/db/connector_response.rs**
  Modify existing code

**crates/router/src/db/customers.rs**
  Modify existing code

**crates/router/src/db/events.rs**
  Modify existing code

**crates/router/src/db/locker_mock_up.rs**
  Modify existing code

**crates/router/src/db/mandate.rs**
  Modify existing code

**crates/router/src/db/merchant_account.rs**
  Modify existing code

**crates/router/src/db/merchant_connector_account.rs**
  Modify existing code

**crates/router/src/db/payment_attempt.rs**
  Modify existing code

**crates/router/src/db/payment_intent.rs**
  Modify existing code

**crates/router/src/db/payment_method.rs**
  Modify existing code

**crates/router/src/db/process_tracker.rs**
  Modify existing code

**crates/router/src/db/queue.rs**
  Modify existing code

**crates/router/src/db/refund.rs**
  Modify existing code

**crates/router/src/db/temp_card.rs**
  Modify existing code

**crates/router/src/lib.rs**
  Modify existing code

**crates/router/src/routes/payment_methods.rs**
  Modify existing code

**crates/router/src/scheduler/workflows.rs**
  Modify existing code

**crates/router/src/types/api/payments.rs**
  Modify existing code