fcl-db: bufdataset: small reprocessing
- adapt method ProcessFieldsToCompareStruct - initialise DBCompareStruct in one place all fields. Used by BuildIndex and Locate.
- add new method BufferOffset - returns offset of buffer data from begining of memory record  = ( sizeof(TBufRecLinkItem) * FMaxIndexesCount )

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23598 3ad0048d-3df7-0310-abae-a5850022a9f2