[TAP] New fix for the transaction management.

The transaction and Statement were closed too early before.
  - Fetching the row was not possible once the first bunch of fetched
rows was over.
  - The problem of "statement is aborted" preventing the re-use of
a same DB connection was apparently still there, but occurred less often.

  Now, any transaction potentially started in a DB connection is always
closed after one of the public functions of JDBCConnection is called ;
except executeQuery(ADQLQuery) whose the call MUST be wrapped inside a
try...catch block in which DBConnection.cancel(true) MUST be called
in case of error (in order to effectively end any started transaction).