Review compile-time constants

[theoreticalbts]

• Make sure all testnet-specific code tests constant IS_TEST_NET
• Make sure all testnet-specific code uses #if[n]def or #if defined()
• Make sure Dockerfile explicitly specifies all "user-facing" configuration parameters
• Document "user-facing" configuration parameters

[theoreticalbts]

The following compile-time constants seem to be related to the p2p code. I don't think any of them should be regarded as "user-facing" so I won't attempt to document them:

• DEFAULT_LOGGER
• ENABLE_P2P_DEBUGGING_API
• P2P_IN_DEDICATED_THREAD
• USE_PEERS_TO_DELETE_MUTEX
• VERBOSE_NODE_DELEGATE_LOGGING

Does our p2p guru @emfrias have any opinion on the matter?

[theoreticalbts]

The remaining options were already documented as part of #487. That means less work for me!

I'm removing all traces of ENABLE_CONTENT_PATCHING from the CMakeLists, Dockerfile and docs. It is not actually used, content patching is always enabled [1].

[1] Except when LOW_MEMORY_NODE is set. Since LOW_MEMORY_NODE throws away comment bodies, content patching doesn't even make sense in the context of LOW_MEMORY_NODE.

[theoreticalbts]

Closing because it was merged.