Change IncrementGenerator to use the Session.Batcher instead of creating a brand new connection to retrieve the next value

See https://nhibernate.jira.com/browse/NH-3591 for details
refactored the tests to make its intention more clear
Merge branch 'fix_increment_generator' of github.com:amroel/nhibernate-core into amroel-fix_increment_generator
IncrementGenerator: Take the opportunity to do some naming cleanup. FirebirdClientDriverFixture: Minor formatting.
Merge fix for NH-3591: IncrementGenerator unnecessarily creates a new connection to the db