Issue #2900, raw database initialization was moved to constructor.
Issue #2900, database record abstract initialized only first time on open.
Import: improved messages
Merge pull request #3026 from henryzhao81/develop

support customized cluster selection
Issue #2900, user instance is cached in immutable field till
 new user will be logged in or permissions will be changed.
fixed link serialization that broke binary compatibility
Merge branch 'develop' of github.com:orientechnologies/orientdb into develop
Issue #2900, compilation error was fixed.
add getDocument() function