Add a guard clause to ExecutionEngine#Aggregate() and Merge()
Merge pull request #155 from mkobetic/engine_guards

Add a guard clause to ExecutionEngine#Aggregate() and Merge()
Refactor server and factors into db package.
Merge branch 'unstable' of https://github.com/skydb/sky into db-refactor
Merge pull request #156 from skydb/db-refactor

Refactor server and factors into db package.
Fix mdb.DBI type.