Revert "fix data corruption in case of partial write (#18 #38)"

This reverts commit ab747ce4682360f8d723e1350d1d8ade65d9b334.
redo the reverted commit; should not try to move the base pointer in case of a full write
do not retry writev if the result was a partial write
redo the partial write impl. (prev commit causes SIGSEGV) (#18 #38)