Implement "lower bound" functionality

[vvish]

Cursor operation MDB_SET_RANGE positions cursor at a first key greater than or equal to specified key.
It is required to implement method in the transaction class to support the functionality.
Method should return ro_view.

[kotov-valery]

@vvish, I would be curious to work on the ticket if that's one is free.

[vvish]

@kotov-valery , thank you.
Reassigned the ticket on you.