fcl-db: bufdataset:
- Introduces RecNo property at TBufIndex level. It enables TBufIndex descendants implement efficient way of getting/setting RecNo.
It also moves "index dependant" code from TBufDataset to TBufIndex and let TBufDataset use methods of TBufDataset.
- Introduces also GetRecord method at TBufIndex level, which provides "index scaning/walking" for TBufDataset

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@26899 3ad0048d-3df7-0310-abae-a5850022a9f2