Bugfixes and improvements

Improved: Added transaction handling - limitation - only read committed
isolation supported
Improved: Responses - UI responsive while fetching row data
Bugfix: Column identifiers will all capital letters and special symbols
(+, -, *, /, etc.) now handled properly
Bugfix: Code completion crashes in specific incomplete query, see
OracleCodeCompletionProviderTest.cs.OracleCodeCompletionTypeTest.TestCodeCompletionTypeWithMissingSelectList
Improved: DB links correctly recognized without domain or with missing
instance definition