Feat: some predefined merge operators

[mindreframer]

Problem:

• currently it's not possible to handle incremental/delta operations on lists / strings / numbers
• everything has to follow the: read / modify / store pattern
• this make certain scenarios hard / impossible to implement properly

Solution:

• provide a list of predefined merge operations
• proper support for lists / counters / binaries / bits
• also same support for column families
• each operation covered with tests

[Elzor]

This file is a part of dev environment