Custom record extractors fail with indexes

[yruslan]

Describe the bug

Under some conditions index offsets do not point to the location of record split when custom record extractors are used.

Expected behavior

Indexes should aways point to the beginning of a next record.

Context

So this is not really a bug. This happens when a custom record extractor uses the input stream in its constructor and the offset reported by it for the first record is not the same as the initial offset of the stream.
While a record extractor should not move the initial offset before the first next() is invoked, it could be nice if the index builder won't depend on this assumption.