fcl-db: bufdataset: add InternalInitFieldDefs for TBufDataset + test (TestFieldDefsUpdate); related also to rev.27021

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@27023 3ad0048d-3df7-0310-abae-a5850022a9f2
fclfcl-db: bufdataset: forgot free filestream and datasetreader


git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@27024 3ad0048d-3df7-0310-abae-a5850022a9f2