Mark LevelDB "Read" and "Exists" functions as const.

Mark the "Read" and "Exists" functions in CLevelDBWrapper as "const".
They do not change anything in the DB, by definition.
Make appropriate getter-routines "const" in CCoinsView.

Mark the "Get"/"Have" routines in CCoinsView and subclasses as "const".
Use const CCoinsView's at some places.

At some places where it is possible (e. g., CheckInputs), use a const
version of CCoinsView instead of a non-const one.
Merge pull request #4561

d0867ac Use const CCoinsView's at some places. (Daniel Kraft)
a3dc587 Make appropriate getter-routines "const" in CCoinsView. (Daniel Kraft)
ffb4c21 Mark LevelDB "Read" and "Exists" functions as const. (Daniel Kraft)