Merge with ColumnFamilies & Hardening CFHandle

ColumnFamilyHandles face the same problem as RocksIterator previously
so used methods were also applied for ColumnFamilyHandles.

Another problem with CF was that Options passed to CFs were
always filled with default values. To enable Merge, all parts
of the database must share the same merge functionality which
is not possible using default values. So from now on every
CF will inherit from db options.

Changes to RocksDB:
- merge can now take also a cfhandle

Changes to MergeTest:
- Corrected formatting
- Included also GC tests
- Extended tests to cover CF related parts
- Corrected paths to cleanup properly within the test process
- Reduced verbosity of the test