Include dependency hierarchy is spaghetti #1105

Today I played around a bit with libclang and made a graph of include dependencies:

https://dev.visucore.com/bitcoin/includes.svg

Edges show a source file that is included into another. You can hover over connections to highlight them, this can help clarify spaghetti parts.

• it would be nice to get rid of headers.h and include only what we need in every compilation unit.
  • Newer files (such as addrman.cpp, protocol.cpp, version.cpp) are more well-behaved in this regard.
• util.h, protocol.h probably pull in too much.
• the hierarchy is too deep; include files shouldn't include that many other include files. For example headers.h←util.h←uint256.h←serialize.h←allocators.h ...
• moving code from .h to .cpp files where possible / efficient would help reduce interdependencies

Agreed on all points, but I vote for this being cleaned up over the next 6 months not in a "change the world" pull request.

Sorry @gavinandresen, see #1108 and #1109...