bdr: conflict handling refactor phase 2

bdr: conflict handling refactor phase 2

New logging API has one unified function that creates Conflict object
which then can be used to log both into server log and to table.

Logging of all conflict types was unified under this new API.

Added two new conflict resolution types 'apply_change' and 'skip_change'
which are used in places where it's impossible to use conflict handlers
(like delete_delete conflict) and also in UDR instead of last update
wins.

In passing fix the timeframe handling of user defined conflict handlers.