Replace `DEFAULT_LOGGER` macro with a variable

Replace the `DEFAULT_LOGGER` macro with a variable.

Related code: https://github.com/bitshares/bitshares-core/blob/9dd8d643d8d55ce5c0dce170dc2c80b0d64e9ff7/libraries/net/node_impl.hxx#L21-L24

Sub-task of #2101 and #1584.

* "#undef" should be used with caution, especially when a "#undef" undefines a macro that was defined in another file.
* Macros should not be used to define constants.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start at libraries/net/node_impl.hxx lines 21-24 and search the repository for DEFAULT_LOGGER references. Review the related scope in issues #2101 and #1584, then verify that the macro has been replaced and the C++ project still builds successfully.

Written by the indexing model from the issue text.