Verification of the use-cases corresponding to `MDB_REVERSEKEY` and `MDB_REVERSEDUP` DB flags enabled

[vvish]

LMDB flags MDB_REVERSEKEY and MDB_REVERSEDUP specify that keys and values are compared (not sorted) in reverse order (from last byte to to the first). Property reversed_flag can be specified to be std::true_type in key or value trait to propagate corresponding flags into lmdb database settings (functionality is already implemented);

It is required to verify the behaviour by integration tests:

• implement traits for std::string with the reversed_flag;
• check that the sorting respects reversed key comparison order.

[vvish]

Should be done after #5