Export to alignment using block read from DB

Currently there is a restriction on import of a few large sequences (~100Mb) from sequence view to msa. Although say 10 such sequences can be examined simultaneously in sequence view, there is a lack of memory required to merge them to msa.
One of the reason of such behaviour is that during the export all the sequences are loaded into RAM.

The first step to the make possible exporting to msa large files It to do the same operations using database block reading, not using getWholeSequence