Replace DEFAULT_LOGGER macro with a variable

[abitmore]

Replace the DEFAULT_LOGGER macro with a variable.

Related code:

bitshares-core/libraries/net/node_impl.hxx

Lines 21 to 24 in 9dd8d64

	#ifdef DEFAULT_LOGGER
	# undef DEFAULT_LOGGER
	#endif
	#define DEFAULT_LOGGER "p2p"

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