compile without warnings

[zeuner]

I cleaned up the code a bit in order to compile warning-free on gcc 6.0. This should be helpful for better maintainability and portability.

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This change is 

[davidediger]

@zeuner Might take us a little bit to review all of this, but thanks for the help. Standby...

[zeuner]

Inform me if anything is unclear.

Some remarks:

• Might be good to document intents regarding const qualifiers - most warnings were due to const correctness

• There may be better ways to ensure strict pointer aliasing (lib/stinger_core/src/stinger.c, stinger_outdegree_get)