Merge branch '19-implement-database-operations' into 'master'

...rc/main/java/de/uniwue/swp/sshtool/database/DBAccess.java

...ain/java/de/uniwue/swp/sshtool/database/DBConnection.java

...java/de/uniwue/swp/sshtool/database/DBConnectionImpl.java

...se/src/main/java/de/uniwue/swp/sshtool/database/File.java

...src/main/java/de/uniwue/swp/sshtool/database/Session.java

...main/java/de/uniwue/swp/sshtool/database/Transaction.java

...opment/database/src/main/sqlQueries/cancelTransaction.sql

...nt/database/src/main/sqlQueries/getFilesInTransaction.sql

...atabase/src/main/sqlQueries/getIncompleteTransactions.sql

development/database/src/main/sqlQueries/getSessions.sql

...atabase/src/main/sqlQueries/getSessionsForTransaction.sql

...database/src/main/sqlQueries/getTransactionForSession.sql

...opment/database/src/main/sqlQueries/updateFilesStatus.sql