Replace globals with domain-specific structs

[jmjatlanta]

Globals are convenient, but lead to problems in maintainability.

• Their use can make two formerly unrelated modules dependent on each other
• They are difficult to document (little context)
• They make testing difficult (dependencies)

The Concept

Divide the globals into structures that are shared with the components that need them. Example (divisions and names will change):

• ChainParameters contains the parameters that control the blockchain (taken mostly from config file and command line)
  • ConsensusParameters may be needed, or may be able to be wrapped into ChainParameters
• ChainStatus contain the mutable data about the current chain at present (i.e. current height, mempool)
• P2PParameters contains the parameters on how the P2P network should be managed
• P2PStatus contains the mutable data about the current P2P network (i.e. connected CNodes)
• RPCParameters contains the parameters that control client connections
• RPCStatus contains the mutable data about current RPC connections

Why go through all this?

Dividing the globals into these structures provides

• Context - You know what area you are talking to, and have narrowed the scope of what you may be affecting by changing a value or method implementation (interfaces become more clearly defined). Example: It is easier to make descriptive variable names when the context is narrowed.
• Testability - Starting X nodes and Y wallets becomes possible within 1 executable, whereas starting 2 nodes within 1 gtest is currently impossible.
• Reuse - Derivative chains (or future main chains) have a better idea of what must be added/removed/changed to provide the desired functionality
• Itterative development - Structures can be implemented with limited functionality that grows over time.

The over-reaching goal is to develop components that have a well-defined interface and push complexity to lower layers (Why are main.cpp and init.cpp so big? Because the complexity is in the wrong place.).

[jmjatlanta]

This could be considered an expansion of the CChainParams struct and Params() function. Note how Params() can be manipulated to make testing possible.