Refactored Query#update so that raw conditions can be merged
Refactored Query#update so that raw conditions can be merged

* Reduced amount of duplicate work than Query#update needs to perform
  to merge another query in.
* Refactored Query#initialize so conditions merging is handled by
  a separate method, shared with #update
* Simplified Query#relative to use Query#merge rather than creating a
  new Query using options.

[#1084 state:resolved]

basex/ardm-do-adapter,datamapper/dm-do-adapter